IS2.3 






Class JHE3S1 


Book_ Pq5_ 

Copyright N?_ \ 9 


COEffilGHT DEPOSIT. 
















SPECTACLES 

AND 

EYEGLASSES 


PHILLIPS 




SPECTACLES 


AND 


EYEGLASSES 

THEIR FORMS MOUNTING AND 
PROPER ADJUSTMENT 

K \ 

.. * by 

R. J. PHILLIPS, M. D. 

OPHTHALMOLOGIST PRESBYTERIAN ORPHANAGE, LATE ADJUNCT PROFESSOR OF DIS¬ 
EASES OF THE EYE, PHILADELPHIA POLICLINIC AND COLLEGE 
FOR GRADUATES IN MEDICINE, ETC. 


1 

> ) 



FIFTH EDITION, REVISED 

WITH 61 ILLUSTRATIONS 


PHILADELPHIA 

P. BLAKISTON’S SON & CO. 


1012 WALNUT STREET 



COPYRTCHT, 1923, BY P. BlAKISTOn’s SON & Co. 


< 




©C1A698508 


PRINTED IN U. S. A. 

BY THE MAPLE PRESS YORK PA 

MAH *■§ 23 




PREFACE TO THE FIFTH EDITION 


In the four previous editions of this treatise the state¬ 
ment that no optical glass was made in America stood 
unchanged. The war has altered^this matter and in.this 
edition is given a short account of the production of such 
glass. 

The toric lenses, which were described in the first edition 
though they were not then commercially obtainable, have 
become of first importance. The sections dealing with 
their manufacture and supply have been rewritten, as 
have those on the newer forms of bifocal lenses. 

In this revision the aim has been to keep step with the 
permanent progress of the arts of the optician rather than 
to pursue subjects of temporary vogue or novelty. 

My thanks are due to Mr. A. Reed Mclntire and to Mr. 
William L. Wall, on whose practical knowledge as manu¬ 
facturing opticians I have at all times been able to draw. 



PREFACE TO THE FIRST EDITION 

Ihis little work is the outgrowth of the instruction on 
the subject of prescribing spectacle frames which has been 
given to successive classes at the Philadelphia Polyclinic 
and College for Graduates in Medicine. The book, like 
the teaching referred to, is intended to supplement studies 
in refraction, and to give the student that knowledge of 
the correct placing of the glasses before the eyes without 
which the most painstaking measurement of the refraction 
will frequently fail of practical results. With the populari¬ 
zation, as one may call it, of ophthalmology in the pro¬ 
fession, many physicians who prescribe glasses are 
compelled, by the lack of skilled opticians in their 
neighborhood, to themselves furnish the spectacles to the 
patient. lo these, it is believed, the knowledge which I 
have endeavored to impart in these pages will prove 
especially useful. 

Of late years much advance has been made in the art of 
making efficient, comfortable and handsome contrivances 
for holding glasses before the eyes, and the increased use 
of prismatic and cylindrical lenses has given the fitting of 
the frames increased importance. Text-books of refrac¬ 
tion remain, however, almost devoid of reference to the 
subject, the scant literature of which is scattered through 
opticians’ trade publications and a few medical periodicals. 
Free application has been made to such sources, and the 
indebtedness incurred duly acknowledged in the text. 

My thanks are due to my friend and instructor, Dr. 
Edward Jackson, for many valuable suggestions in writing 
this treatise, and, indeed, for directing my attention to the 
need of a book on spectacles. 

•• 

Vll 


Vlll 


PREFACE TO THE FIRST EDITION 


Dr. George M. Gould kindly furnished me with some 
references used in the introduction, and I am indebted to 
Messrs. Wall & Ochs, Bonschur & Holmes, and J. W. 
Queen & Co. for a number of cuts. 


CONTENTS 


Page 

Introduction. . 

I. General Considerations. g 

The Material of Frames. g 

The Component Parts of Spectacles.n 

Tire Lenses: Their Material and Manufacture.1.2 

Eye Wires, Temples, and Bridges.22 

The Different Patterns of Spectacles.22 

Bifocal Glasses.27 

The Varieties of Eyeglasses.32 

Spectacles for Cosmetic Effect.36 

II. The Principles of Spectacle Fitting.38 

Centering and Decentering.38 

Prismatic Effect of Decentering.40 

Normal Lateral Centering.43 

Normal Vertical Centering.44 

Distance of the Glasses from the Eyes.45 

Perpendicularity of the Plane of the Lenses to the Visual Axes. 46 
Periscopic Glasses.49 

III. Prescription of Frames.52 

The Measurements Required.52 

Obtaining the Interpupillary Distance.55 

Height of the Bridge.57 

Relation of the Top of the Bridge to the Plane of the Lenses . . 58 

Width of Base. # .59 

Angle of Crest of Bridge.59 

Prescription of Eyeglasses.61 

IV. Inspection and Adjustment of Spectacles and Eyeglasses . 63 

Proving the Strength of Lenses . ..63 

Phacometers. 64 

Neutralization of Spherical Lenses.65 

Neutralization of Cylindrical Lenses.66 

Neutralization of Sphero-cylindrical Lenses.68 

Locating the Optical Center.68 


IX 
































X 


CONTENTS 


Page 

Finding the Apex of a Prism.69 

Measuring the Strength of a Prism. 70 

Detection of Scratches, Specks, Flaws, Etc.76 

Irregularity of the Refracting Surfaces.76 

Adjusting Spectacle Frames.76 

Adjustment of Eyeglasses.82 

The Care of Spectacles.86 

Index .g 7 


I 










Fig. 


LIST OF ILLUSTRATIONS 


Page 


1. Position of the parts of spectacles. 

2. Position of the parts of eyeglasses. 

3. Sections of lenses. .. 

4. Optician’s lens-grinding lathe. 

5. Tool for grinding convex tores .. 

6. Tool for grinding concave tores. 

7. Frameless spectacles. 

8. Forms of spectacle bridges. 

9. Bridge for removing pressure. .. 

10. Ovals, showing the shapes of spectacle eyes. 

11. Forms of bifocal glasses. .. 

12. Patterns of lenticular segments. 

13. Mounted one-piece bifocals. 

14. Extra front. 

15. Frameless eyeglass. 

16. Oxford style eyeglass. 

17. Coil-spring eyeglass. 

18. Bar-spring eyeglass. 

19. Finger-piece mounting. 

20. Patterns of the offset guard. 

21. Eyeglass studs. 

22. Spectacles with lenses decentered “in,”. 

23. Section of a normally centered lens. 

24. Decentered lens, showing prismatic effect. 

25. Natural position of the spectacle bridge. 

26. Top of bridge “out” from plane of the lenses. 

27. Top of bridge “in” from plane of the lenses. 

28. Spectacles facing directly forwards. 

29. Spectacles facing downwards and forwards. 

30. Spectacles facing inwards. 

31. Front and back of a convenient spectacle rule. 

32. Method of measuring the height of a spectacle bridge. . 

33. Simplest method of measuring the interpupillary distance. 

34. Maddox pupil localizer. 

35. Method of using the Maddox pupil localizer. 

36. A common form of pupillometer.. 


11 

12 
16 

19 

21 

21 

23 

23 

25 

26 

27 

28 

30 

32 

33 
33 
33 

33 

34 

35 

36 

39 

40 
40 

45 

46 

46 

47 
47 
43 
53 

55 

56 
56 

56 

57 


xi 






































xii LIST or ILLUSTRATIONS 

Fig. Page 

37. Method of measuring the distance of the bridge “out”.58 

38. Method of measuring the distance of the bridge “in”. ..... 59 

39. Implement for measuring angle of crest of bridge.61 

40. Use of a crest measure . ..61 

41. Mr. Brayton’s lens measure.65 

42. Apparent displacement of lines caused by rotating a cylinder ... 67 

43. Method of finding the axis of a cylindrical lens.67 

44. Ready method of locating the optical center of a lens.69 

45. Mode of marking the apex of a prism. 7 ° 

46. A prism improperly held.7° 

47. Method of determining the deviating angle of prisms. 74 

48. Optician’s pliers. 75 

49. Jaws of Pliers.'. 75 

50. Rotation of a lens within its eye wire.76 

51. Bend at the junction of the eye wire and bridge.76 

52. Showing the planes of the lenses crossing each other. 77 

53. Bend of the bridge.78 

54. Inequality of corresponding angles of the bridge.78 

55. Angles on one side of the bridge too small; on the other too large. 78 

56. Proper fitting of hook temples.80 

57. A common but incorrect shape of the nose-pieces of eyeglasses. . 82 

58. Eyeglasses with nose-pieces of correct shape.82 

59. Diagram showing inclination of nose-pieces.83 

60. Diagram showing inclination of nose-pieces.84 

61. The points at which eyeglasses are to be adjusted.85 





















SPECTACLES AND EYEGLASSES 


INTRODUCTION 

At what time man invented lenses and discovered the aid 
which they are capable of lending to vision is a matter 
beyond our knowledge. It is tolerably certain that they 
were known to civilizations earlier than ours. Though it 
might be difficult to prove that spectacles were known to 
the ancients, the evidence in relation to their acquaintance 
with the essential element of spectacles, the lens, is reason¬ 
ably convincing. This evidence was, for the most part, 
discovered by Sir Austen Henry Layard among the ruins of 
old Nineveh, and is of the most interesting character. 
Among the articles which he unearthed was a specimen of 
transparent glass (a small vase or bowl) with a cuneiform 
inscription fixing its date quite accurately to the latter 
part of the seventh century B. C. (“Discoveries Among 
the Ruins of Nineveh and Babylon, etc.,” by Austen H. 
Layard, New York, 1853, P- 196-) This is the most 
ancient known specimen of transparent glass, though 
Egypt furnishes it of a date only a century later, and opaque 
or colored glass was manufactured at a much earlier 
period, some specimens of the fifteenth century B. C. still 
enduring. However, the ancient nations were not com¬ 
pelled to wait for transparent glass in order to invent 
lenses, as they had in rock crystal a material admirably 
adapted to that purpose, and Layard was so fortunate as to 
discover such a lens in Nineveh. {Ibid., p. 197.) Sir 
David Brewster, who examined this lens, described it 
as being plano-convex, of a diameter of one and a half 


2 


SPECTACLES AND EYEGLASSES 


inches, and capable of forming a tolerably distinct focus 
at a distance of four and a half inches from the plane 
side. It is interesting to note further in regard to this, 
the oldest lens in existence, that it is fairly well polished, 
though somewhat uneven from the mode in which it 
was ground, which Brewster concludes was not upon a 
spherical surface, but by means of a lapidary’s wheel, or 
some method equally rude. Another evidence of the use 
of lenses has come down to us from antiquity. Upon 
record cylinders of old Nineveh, and on engraved gems and 
stones of Babylon, Egypt, and other sources which long 
antedate the Christian era, are characters and lines of such 
delicacy and minuteness as to be undecipherable without 
the aid of a magnifying lens. Taking these facts in con¬ 
junction, the statement that some of the properties of 
lenses were known to and utilized by the ancients, the old 
record writers of Assyria, for instance, may be regarded as 
almost as well demonstrated as though it were made of a 
modern engraver, and we were to step into his workshop 
and find his magnifying loup lying beside his work. 

The testimony as to their use by the Romans during 
their supremacy is of a less conclusive character. The 
statement frequently made that the Emperor Nero used a 
concave jewel to assist his sight rests upon some obscure 
sentences in Pliny. That author says: “Nero could see 
nothing distinctly without winking and having it brought 
close to his eyes.” (Bk. n, Chap. 54, Riley’s Trans.) In 
another place, speaking of the emerald, smaragdus, he 
says: “In form these are mostly concave, so as to 
reunite the rays of light and the powers of vision. * * * 
When the surface of the smaragdus is flat, it reflects the 
image of objects in the same manner as a mirror. The 
Emperor Nero used to view the combats of gladiators upon 
(with, or by means of) a smaragdus.” (Bk. 37, Chap. 17.) 
The mention of the reflecting properties of the emerald 
immediately before the statement of Nero’s use of it, with 


INTRODUCTION 


3 


the alternative renderings of the Latin ablative, smaragdo, 
make the supposition that Nero used the emerald as an 
eyeglass uncertain, though in view of his clearly described 
nearsightedness, the conjecture is probable enough. 

Lenses appear to have been unknown in Europe during 
the first twelve hundred years of the Christian era, though 
the Saracen Alhazen, who died in Cairo in 1038, has left 
books showing his acquaintance with them. These books 
were brought to Europe at a very early period, and the 
manuscripts yet exist, some in the Bodleian library, and 
another portion in that of the University of Leyden. 
They treat with remarkable clarity and accuracy of the 
laws of reflection and refraction, including reflection 
and refraction by surfaces convex, concave and cylin¬ 
drical. Some of their diagrams showing the course of 
light rays are in use in our text-books at this day. In 
spite of some errors, they exhibit, also, a good knowledge 
of the anatomy and physiology of the eye. It was prob¬ 
ably from these works that the early writers obtained 
their first hints of the science of optics, on the revival of 
learning in the fourteenth and fifteenth centuries. In 1572 
a Latin translation of Alhazen’s treatise on optics was 
published at Basle, and in 1600 Johann Kepler, the 
astronomer, wrote the first European work on the subject. 
It is worthy of note that Alhazen was born at Bassora, at 
the head of the Persian Gulf and less than five hundred 
miles from the spot where, sixteen hundred years before, 
had stood the palace of the Assyrian kings in the ruins of 
which Sir Henry Layard found the lens of crystal. It 
might, perhaps, be plausibly maintained that in the coun¬ 
tries about the Tigris some knowledge of optics, and of 
convex lenses, has persisted without eclipse from the most 

remote ages. 

We are told in a general way that the Chinese have for 
ages employed spectacles for the relief of defective eye¬ 
sight. This is, perhaps, to be regarded as only another 


4 


SPECTACLES AND EYEGLASSES 


instance of the exercise of that claim to priority which the 

« 

Chinese are known to extend over every good and perfect 
gift. It is known, however, that the countries about the 
Mediterranean, Greece, Rome and Palestine, each in its 
day of grandeur, had some intercourse and trade with 
China. Europe undoubtedly received the science of optics 
in a fairly advanced state from the Arabs. The Chinese 
may very well have obtained their knowledge from the 
same source. So far as is shown by the evidence, the 
ancient Assyrian was the first to make and use a lens, and 
the Arab the first to write a scientific treatise on optics. 

The earliest European reference to our subject occurs in 
the writings of Roger Bacon, who died in 1292, and to 
whom the invention of the instrument he describes is 
sometimes accredited. Bacon’s glass was apparently a 
large plano-convex lens, probably what we now call a 
reading glass, intended to be held in the hand, and of it he 
says: “This instrument is useful to old men and to those 
that have weak eyes; for they may see the smallest letters 
sufficiently magnified.” Spectacles proper—that is, glasses 
mounted so as to retain themselves upon the face—appear 
to have been invented in Florence some time between 
1280 and the close of the thirteenth century. Dr. Samuel 
Johnson is said to have expressed surprise that the inventor 
of such useful articles has found no biographer. Doubtless 
among the thousands for whom the discovery has kept 
open the sources of knowledge there would be found one 
to pay this tribute to the fame of his benefactor were the 
identity of the latter a matter of certainty. But, unfortu¬ 
nately, our evidence on the point is of the most fragmen¬ 
tary character. The tomb of Salvinus Armatus, a 
Florentine nobleman who died in 1317, is said to bear an 
inscription to the effect that he was the inventor. If 
epitaphs enjoyed a less equivocal reputation for truthfulness 
he would doubtless be held in grateful remembrance as the 
man who has lengthened youth by postponing old age; 



INTRODUCTION 


5 


and, like Joshua, kept back the night until the day’s work 
was done. 

Whoever the inventor, Alessandro di Spina, a monk of 
Florence who died in 1313, is generally accredited with 
having made public the use of spectacles, and by several 
Florentine writers of that time we find them mentioned 
and recommended. Pissazzo, in a manuscript written in 
1299, says: “I find myself so pressed by age that I can 
neither read nor write without those glasses they call 
spectacles, lately invented, to the great advantage of poor 
old men when their sight grows weak.” Friar Jordan, of 
Pisa, in 1305 says that “it is not twenty years since the 
art of making spectacles was found out, and is, indeed, one 
of the best and most necessary inventions in the world.” 

An early mention of spectacles, or, in the language of 
that time, “a spectacle,” occurs in “The Canterbury 
Tales,” where Chaucer makes the Wife of Bath use the 
metaphor:— 


Povert (poverty) full often when a man is lowe, 

Makith him his God and eek himself to knowe. 

Povert a spectacle is, as thinkith me, 

Through which he may his verray (true) frendes se. 

There is in existence in the church of Ogni Santi, 
Fifcrence, an old fresco by Domenico Ghirlandajo, repre¬ 
senting St. Jerome, and dated 1480. The Saint is por¬ 
trayed seated at a desk, apparently deep in the composition 
of one of the blasts against the Heretics for which he was 
famous. Upon a peg at the side of the desk, together with 
the ink-horn and a pair of scissors, hangs a small handleless 
pince-nez. The glasses are round and framed in dark bone, 
and in the bridge, also of bone, is a hinge. /Though the 
artist seems to have been little impressed by the fact that 
St. Jerome died in the year 420, nearly nine centuries 
before spectacles were invented, the mounting and material 
represented in these early spectacles are worthy of note as 


6 


SPECTACLES AN1) EYEGLASSES 


showing their form in Ghirlandajo’s time, and probably 
that in which they originated. 

In the early references to spectacles it is the convex lens 
for the use of the presbyopic which is mentioned. It 
must have been early discovered that there is a more or 
less close relation between the age of the wearer and the 
strength of the convex glass required, and the baneful 
theory was soon developed that this relation is constant, 
and that it would be ruinous to use a lens “too old for the 
eyes,” a superstition from which the public is even yet not 
fully emancipated. We find it rampant in Pepys’ time, 
preventing his oculist, Dr. Turberville,* from giving that 
gentleman a proper correction for his accommodative 
asthenopia, of which the diary gives an accurate picture, 
and losing to the world many a priceless page. Pepys 
sa ys (June 30, 1668): “My eyes bad, but not worse, only 
weary with working. * * * I am come that I am not 
able to read out a small letter, and yet my sight good, for 
the little while I can read, as ever it was, I think.” But 
Dr. Turberville warns him against'glasses too old for him, 
and so the diary is closed, and Pepys in a last pathetic 
entry resigns himself to coming blindness; and yet the 
convex lenses were at his hand, ready to dissipate the mists 
before him and enable him to “gaze upon a renovated 
world.” > 

Improvement in spectacles appears to have been slow. 
The world waited more than two centuries after Kepler 
for another signal advance. Sir David Brewster is said to 
have discovered his own astigmatism; that is, he dis¬ 
covered that vertical and horizontal lines were not equally 
well seen by him at like distances, but the phenomenon 
was not explained and the observation faded from view. It 
remained for George Airy, the astronomer, to rediscover 

* Daubigny Turberville; created M.D. at Oxford in 1660. He practiced 
with great reputation as an oculist in London. His monument yet remains 
in Salisbury Cathedral, where he was buried. 



INTRODUCTION 


7 


astigmatism, which he did about 1827, to determine that 
the curvature of the cornea was greater in one diameter 
than in another at right angles to the first, and to apply the 
cylindrical lens to the correction of the condition. Mr. 
Airy’s right eye was myopic, while in the left he had com¬ 
pound myopic astigmatism. By a careful comparison of 
the appearance of objects when viewed with each eye 
singly, and a study of the effect of concave lenses held 
before the left eye upon lines crossing each other at right 
angles, he was able to conclude that the refraction of that 
eye differed in different planes. Mr. Fuller, an optician 
of Ipswich, made, under Airy’s direction, a concave sphero¬ 
cylindrical lens which satisfactorily corrected his refrac¬ 
tive error. Thus was the last great discovery in spectacles 
accomplished—a bit of work for completeness leaving 
nothing to be desired, and of not sufficiently acknowledged 
importance to humanity. 

Benjamin Franklin invented bifocal spectacles. Since 
this statement is supposed by many to rest on tradition 
only, it may be of interest to quote a portion of a letter of 
Franklin’s which bears upon the point. The letter is 
addressed to George Whately, of London, and is dated 
Passy, 23d May, 1785. In it Dr. Franklin says: “By 
Mr. Dolland’s saying that my double spectacles can only 
serve particular eyes, I doubt he has not been rightly 
informed of their construction. I imagine it will be found 
pretty generally true that the same convexity of glass 
through which a man sees clearest and best at the distance 
proper for reading, is not the best for greater distances. I 
therefore had formerly two pairs of spectacles, which I 
shifted occasionally, as in traveling I sometimes read, and 
often wanted to regard the prospects. Finding this change 
troublesome and not always sufficiently ready, I had the 
glasses cut and half of each kind associated in the same 
circle. By this means, as I wear my spectacles constantly, 
I have only to move my eyes up or down, as I want to 


8 


SPECTACLES AND EYEGLASSES 


see distinctly far or near, the proper glasses being always 
ready. This I find more particularly convenient since my 
being in France. * * *” (“The Complete Works of 
Benjamin Franklin.” Ed. by Johh Bigelow, New York, 
1888.) 

We may infer from the context that the invention took 
place before Franklin went to France, which was in the 
latter part of 1776. As he was born in 1706, the necessity 
for a double glass would first arise about 1750, and the 
invention therefore took place some time between this 
date and that of the journey to France. 

The frames in which spectacles were mounted continued 
to be very clumsy affairs until the beginning of the last 
century, when light metal frames were introduced in place 
of the earlier devices of bone, horn, or shell. Their later 
evolution has generally been along the lines of improved 
mechanical construction and increased lightness and 
beauty. It would be difficult to mention an article which 
plays a more important part in modern life than do 
spectacles, or one which plays its part more acceptably. 
It is scarcely possible to estimate them at their true worth, 
or to imagine our condition without them. Deprived of 
their aid, most men would be too old for work at fifty, 
and purblind at sixty. For us all, as an old writer quaintly 
observes, “they keep the curtain from falling until the 
play has come to an end.” 


I. GENERAL CONSIDERATIONS 


By far the most generally useful method of placing 
glasses before the eyes is by spectacle frames, though the 
eyeglass, or pince-nez , has advantages in some cases, from 
the facility with which it may be placed in position or 
removed. The superiority of eyeglasses in appearance 
is another point not unworthy of consideration, as the 
glasses will surely be more constantly worn if they are 
becoming than if they are not so. Moreover, the patient 
is justly entitled to the correction of his refractive error 
with as little injury to his appearance as possible. The 
disadvantages of eyeglasses are, that for constant wear 
they are seldom so comfortable as spectacles; that on some 
faces it is nearly impossible to keep them in place; while, 
where the contained glass is cylindrical or prismatic, the 
rotary displacement which it is possible for the glass 
to take is a serious and sometimes fatal objection to their 
adoption. 

Lorgnettes and single eyeglasses, or quizzing glasses, as 
they are called, are little more than playthings; though 
sometimes, as in aphakia, or high myopia, a strong convex 
or concave lens in one of these forms is of use when the 
spectacles constantly worn do not give the vision which 
may occasionally be required. 

The Material of Spectacle Frames is usually gold, silver, 
steel, brass gold plated, or alloys containing nickel and tin. 
Horn, tortoise shell and celluloid find, or have found, a 
limited applicability in this connection. 

Gold, of from io to 14 karat, is, by far, the best material 
for frames. Finer than this it is too flexible, while if less 
pure it may blacken the skin. In the end, such frames 

9 


IO 


SPECTACLES AND EYEGLASSES 


are cheaper than steel, as, owing to the liability of the 
latter metal to rust when in contact with the moist skin, 
the gold will outlast it many times over. In eyeglasses, 
however, the parts are heavier, and the metal is not in 
contact with the skin; so that there is not the same liability 
to rust. The gold frames furnished by opticians in this 
country usually have a stamp mark on the inner side of the 
right temple, near the hinge, which denotes the fineness 
of the gold: thus 8 karat is marked +; io karat, 0; 12 karat, 
*; while 14 karat, or finer, is marked 14k, etc. “Gold 
filled” frames of the best quality and rather heavy stock 
are fairly rigid and durable and are considerably used, 
especially for children’s glasses. 

Silver and brass are very poor material for frames, 
being soft, flexible and entirely lacking in elasticity. 
They are only useful for workmen’s protective goggles 
or some such purpose where very heavy frames are allow¬ 
able. The various alloys of nickel and tin, sold under 
trade names, have the faults of silver and brass in less 
degree. Some of them make fairly satisfactory frames for 
eyeglasses. For spectacles they are scarcely worth con¬ 
sidering. 

Gold is, therefore, not only the best but the only good 
metal for frames. A cheap frame having a certain 
amount of rigidity and elasticity, and free from liability 
to rust, is still a thing to be desired. Aluminum has been 
regarded as promising in this connection, but except light¬ 
ness it has scarcely a quality to recommend it. It is 
soft, flexible and inelastic, and is readily corroded by the 
perspiration if the latter happen to be alkaline, which it 
frequently is. Moreover, aluminum cannot be soldered. 
In itself, therefore, it is unlikely ever to be available for 
this purpose. Some of its alloys, however, have interest¬ 
ing properties. That composed of ten parts of aluminum 
and ninety parts of copper some authorities assert to be the 
most rigid metal known. It is a red gold color, does not 


GENERAL CONSIDERATIONS 


II 


tarnish readily, is lighter than brass or gold, and can be 
soldered. Tt is possible that in some such alloy will be found 
a material having the valuable properties of gold for this 
purpose and without the latter’s cost. Largely in response 
to a demand for novelty celluloid or zylonite frames have 
been made in great variety. This substance is inflammable 
but can be bent in boiling water. As it is rather brittle, 
especialy when exposed to low temperatures, it is necessary 
to use it in thick pieces, but it is light in weight. In 



Pig. i. 


irames made wholly of this material it is nearly impossible 
to make any adjustment of the bridge, and rivets and 
screws do not hold very well. To meet these objections 
frames with metal hinges, bridges and temples, and 
finally metal frames covered with celluloid have appeared. 
The all celluloid frame with metal hinges appears to be the 
most popular one. With straight or half hook temples 
it is useful for glasses which are worn only in-doors, or for 
near work. 

The Component Parts of Spectacles— A pair of spec¬ 
tacles is made up of fifteen or seventeen pieces, whose 
positions are shown in Fig. i. They are: two lenses, two 
eye wires, four end pieces, two screws, two pins, or dowels, 
two temples, and one bridge. Sometimes the rings upon 



12 


SPECTACLES AND EYEGLASSES 


the temples, through which the dowels pass, are formed as 
separate pieces. Fig. 2 shows the name and position of each 
part of an eyeglass. A glance at the more important of the 
many interesting processes required in making these differ¬ 
ent parts will contribute to an understanding of the subject. 

The Lenses. —The word lens is the Latin name of the 
lentil, a small bean. The resemblance in shape caused the 
name to be applied to the optical implement. Spectacle 
lenses are usually made of glass; sometimes of rock crystal 
(crystallized quartz). The latter substance has a slightly 



higher index of refraction, so that a lens of a given strength 
may be somewhat lighter when made of it than when made 
of glass. The notion is common that these “pebbles,” 
as they are called, possess a peculiar virtue in strengthen¬ 
ing the eyes or in some other direction. I suppose the 
idea is that, being the product of Nature’s laboratory, 
they are necessarily superior. The advantage which 
they may have of being slightly lighter and harder than 
lenses of glass is more than counterbalanced by their 
higher cost, and by the fact that the index of refraction 
of rock crystal is not very constant. 

Glass cannot be regarded as a definite chemical com¬ 
pound. It is of many varieties differing widely in compo¬ 
sition. Ordinary glasses are mixtures of silicates, that is; 




GENERAL CONSIDERATIONS 


13 


silicic acid combined with some two or more of the metalic 
bases: sodium, calcium, potassium and lead. Ordinary 
optical glasses are divided into two types: (1) Crown 
glasses, which contain lime as one of their constituents 
and do not contain lead. (2) Flint glasses, which contain 
lead but no lime. A large number of special optical 
glasses have been produced by using other bases such as 
barium, magnesium, aluminum and zinc, either with or 
without the bases mentioned above. These glasses have 
optical properties valuable in various optical instruments 
but are not used in ophthalmic lenses. 

The glass used for nearly all ophthalmic lenses is crown 
glass specially made for the purpose. Its essential quali¬ 
ties are: (1) Transparency and freedom from color. (2) 
Homogeneity. (Freedom from veins or striae of unequal 
refraction.) (3) Hardness and chemical stability. (4) Ab¬ 
sence of internal strain. (Caused by unequal contraction 
of the outer and inner portions of a mass of glass while 
cooling.) (5) Constancy of its optical pro parties of refraction 
and dispersion. To attain these qualities in a high 
degree requires great care in the selection of raw materials 
of constant composition and their thorough admixture 
in constant proportions, as well as skill in the processes 
of melting, casting and rolling into sheets, annealing, cut¬ 
ting into pieces, and moulding into rough blanks for lenses. 

Previous to the European war optical glass was not made 
in America. The curvature of the lensmaker’s tool is 
corelated with the density of the glass which he grinds. 
Hence a market once supplied with a satisfactory glass has 
a great incentive to adhere to its source of supply. This, 
and a little mystery with which the art of making optical 
glass was veiled, served to keep our opticians dependent 
on Europe in this regard. During the war the Govern¬ 
ment lent its aid in finding sources of pure raw materials 
and in experimental work of optical glass making. The 
War Industries Board in the Official Bulletin of June 21, 


14 


SPECTACLES AND EYEGLASSES 


1918, formally announced the successful production of opti¬ 
cal glass in America and gave the Bausch & Lomb Optical 
Company full credit for the pioneer work and achievements. 
Apparently all of our lenses are now ground from 
American glass. Its index of refraction is 1.523. 

Colored Glass. —Colored glasses are produced by adding 
metal oxides to glass while molten, each oxide producing 
a characteristic color. Such glass is graded by depth of 
color as A, B, C, and D; A being the lightest. The neutral 
tint called “smoke” is the result of using a number of the 
metalic oxides together. It is furnished in four shades and 
is the best color for use during mydriasis and in temporary 
photophobia from disease or after operations. Crookes 
glass is of a neutral tint lighter than the lightest shade of 
smoke. This color darkens somewhat, however, with age 
and exposure to light. The glass is an attempt to suppress 
the heat and ultra-violet rays without altering the color 
vision. It is suitable for temporary or occasional use whern 
the eye is exposed to excessive light. 

The two broad, polished surfaces of a lens are called its 
refracting surfaces, since it is at these surfaces that the rays 
of light are refracted when the lens is in use. On the shape 
of these surfaces, and their position relative to each other, 
depend all the powers and properties of a lens. Each of 
these surfaces may be either plane, spherical, or cylindrical. 
A spherical surface is such a one as, continued in all direc¬ 
tions, would form a sphere, and which is, therefore, a seg¬ 
ment of a sphere. Similarly, a cylindrical surface is the 
segment of a cylinder. Spherical and cylindrical surfaces 
may be either convex or cancave. A single surface of a 
lens may be, therefore, either 
Plane, 

Convex spherical, 

Concave spherical, 

Convex cylindrical, 

Concave cylindrical. 


GENERAL CONSIDERATIONS 


1 S 


Since every lens has two refracting surfaces, the list of 
lenses which it is possible for the lens maker to produce 
by combinations of these five primary surfaces is as follows: 

1. Prismatic. 

2. Plano-convex spherical. 

3. Plano-concave spherical. 

4. Plano-convex cylindrical. 

5. Plano-concave cylindrical. 

6. Biconvex spherical. 

7. Biconcave spherical. 

8. Concavo-convex (two varieties): 

(a) Radius of curvature of convex surface greater 
than that of concave. (Converging meniscus.) 

(b) Radius of curvature of convex surface less than 
that of concave. (Dispersing meniscus.) 

9. Sphero-cylindrical (four varieties): 

(a) Convex sphere combined with convex cylinder. 

(b) Convex sphere combined with concave cylinder. 

(c) Concave sphere combined with concave cylinder. 

(d) Concave sphere combined with convex cylinder. 

10. Biconvex cylindrical, axes coincident. 

11. Biconvex cylindrical, axes crossed. 

12. Biconcave cylindrical, axes coincident. 

13. Biconcave cylindrical, axes crossed. 

14. Concavo-convex cylindrical, axes coincident. 

15. Concavo-convex cylindrical, axes crossed. 

Sections of lenses are shown in Fig. 3, each section illus¬ 
trating two or more lenses, accordingly as we regard the 
curved lines as sections of spheres, or cylinders, and the 
straight lines as planes, or as sections of cylinders in 
the direction of their axes. 

Lastly, the prism may be introduced as an element into 
each of these lenses. Thus we have quite a long list of the 
possible forms of the lens, and that without considering the 
“ toric” surface, which will be spoken of later. Of these 
lenses only the nine first mentioned, and the combination 


16 SPECTACLES AND EYEGLASSES 

of some of them with the prism, are in practical use. The 
others, the bicylindrical lenses, besides being difficult of 
manufacture, have each its optical equivalent in some sim¬ 
pler form of lens, either piano-cylindrical or sphero-cylin¬ 
drical. They are only mentioned now because their use 
has been advocated by a few writers in the past. 

Difficulties in grinding, and the near equivalence of cer¬ 
tain of the lenses mentioned among the first nine of our 
list, render the use of some of these lenses quite rare. It 
is, for instance, more difficult to grind a perfect piano- 
spherical lens than it is to grind a bispherical, and as in 
weak lenses, such as are used in spectacles, the action of 


Fig. 3. 





every piano-spherical lens can be nearly exactly duplicated 
by some bispherical one, we seldom find piano-spherical 
lenses in use. Among sphero-cylindrical lenses also it is 
usual to consider certain combinations as equivalent to 
each other. For example, a convex spherical combined 
with a convex cylindrical, as equivalent to some stronger 
convex spherical combined with a concave cylinder. 
These lenses are only strictly equivalent, however, for a 
small area near their optical centers. When their influence 
on the field of vision is taken into account, they can no 
longer be considered identical, as we shall see in consid¬ 
ering periscopic lenses. 

In Fig. 3, the first three lenses shown act as convergers 
of rays, and are all considered as convex, or “plus” lenses, 












GENERAL CONSIDERATIONS 


n 


TABLE I 


j 

OLD SYSTEM 

1 

NEW SYSTEM 

' 

I 

II 

III 

IV 

V 

VI 

VII 

VIII 

No. 

Focal 

Focal 


No. 

Focal 

Focal 

No. 

of the 

Distance 

Distance 


of the 

Distance 

Distance 

Corres- 

Lens, 

in 

in 

Equiva- 

Lens, 

in 

in 

ponding 

Old 

English 

Mini- 

lent in 

New 

Milli- 

English 

of the Old 

System 

inches 

meters 

Diopters 

System 

1 

meters 

inches 

System 

72 

67.9 

1724 

0.58 

0.25 

4000 

157.48 

166.94 

6o 

56.6 

1437 

0.695 

0.5 

2000 

78.74 

83.46 

48 

45-3 

1150 

0.87 

o. 7 S 

1333 

52.5 

55-63 

42 

39.6 

1005 

0.90 

1 

1000 

39.37 

41-73 

36 

34 

863 

1.16 

1.25 

800 

31.5 

33 39 

30 

28.3 

7 i 8 

1-39 

i .5 

666 

26.22 

27.79 

24 

22.6 

574 

1.74 

1 -75 

57 i 

22.48 

23.83 

20 

18.8 

477 

2 .09 

2 

500 

19.69 

20.87 

18 

17 

431 

2.31 

2.25 

444 

17.48 

18.53 

16 

15 

381 

2.6 

2.5 

400 

15.75 

16.69 

IS 

14.1 

358 

2.79 

3 

333 

13.17 

13-9 

14 

13.2 

335 

2.98 

3-5 

286 

11.26 

11.94 

13 

12.2 

312 

3.20 

4 

250 

9.84 

10.43 

12 

11.2 

287 

3.48 

4-5 

222 

8.74 

9.26 

11 

10.3 

261 

3.82 

5 

200 

7.87 

8.35 

10 

9-4 

239 

4.18 

5-5 

182 

7.16 

7.6 

9 

8.5 

216 

4.63 

6 

166 

6.54 

6.93 

8 

7-5 

190 

5.2s 

7 

143 

5.63 

5-97 

7 

6.6 

167 

5.96 

8 

125 

4.92 

5-22 

6 H 

6.13 

155 

6.42 

9 

III 

4-37 

4.63 

6 

5-6 

142 

7.0 

10 

100 

3-94 

4.17 

sH 

5-2 

132 

7.57 

II 

91 

3.58 

3-8 

5 

4-7 

119 

8.4 

12 

83 

3-27 

3-46 

4 H 

4.2 

106 

9.4 

13 

77 

303 

3-21 

4 

3.8 

96 

10.4 

14 

71 

2.8 

2.96 

3 H 

3-3 

84 

II.9 

15 

67 

2.64 

2.8 

3 H 

3-1 

79 

12.7 

l6 

62 

2.44 

2.59 

3 

2.8 

71 

14.0 

17 

59 

2.32 

2.46 

2 H 

2.6 

66 

15.1 

18 

55 

2.17 

2.29 

2 H 

2.36 

60 

17.7 

20 

50 

1.97 

2.09 


2.1 

53 

18.7 





2 

1.88 

48 

20.94 






being designated by the sign +, or sometimes by cx. 
The remaining lenses in the figure act as dispersers of 
the rays and are known as “minus,” or concave lenses, 
and receive the sign —, or sometimes cc. For the terms, 

spherical lens, cylindrical lens, prismatic lens, sphero-cylin- 

2 





























x8 SPECTACLES AND EYEGLASSES 

drical lens, etc., the words sphere, cylinder, prism, 
sphero-cylinder, etc., are frequently employed and are 
unobjectionable. Finally, the sign O is used for com¬ 
bined with” in the formula of a combination lens, as + 4. 
sphere O + 2. cylinder. 

The new system of numbering lenses, the dioptric system, 
has so entirely fulfilled the requirements of the users of 
lenses, and has so simplified and facilitated our every-day 
work and calculations, that the old or inch system of 
numbering is rapidly becoming of historical interest only. 
As its use, however, still survives in certain quarters, 
and lenses are frequently met with which are marked 
by this system, a table showing the equivalence of the ordi¬ 
nary lenses of the test case in the two systems is shown on 
page 17. It is calculated for an index of refraction of 1.53* 

The simple apparatus used for grinding a single spherical 
lens is shown in Fig. 4. The disk of glass of which a lens 
is to be made is fastened, by means of pitch, to a small, 
cubical block of iron having a pit in the surface opposite 
that to which the glass is fastened. Into this pit fits a 
pin upon a lever, which is in the hand of the workman. 
When the free surface of the glass is applied to the surface 
of the “tool” to whose form it is to be ground, it, together 
with the block of iron, turns upon the pin. The universal 
joint at the end of the lever permits lateral and vertical 
movements, so that the workman is able to carry the glass 
freely over all portions of the tool. 

The tool which gives the shape to the surface of the 
glass is made of steel; and for spherical glasses is in the 
form of a disk, with its surfaces looking upward and down¬ 
ward, and revolving about a vertical axis, like a potter’s 
wheel. The upper surface of this disk is convex for grind¬ 
ing concave glasses, or concave for grinding convex glasses. 
Of course, each strength of lens requires a separate tool 
having the requisite convexity or concavity of surface. 
The abrading material placed upon the surface of the tool 


GENERAL CONSIDERATIONS 


19 


is wet, powdered emery of successively finer and finer 
grades until the desired amount of glass has been ground 
away. When this process is complete, the surface of the 
glass has the desired spherical curvature, but it is rough: 
that is, it is “ground glass.” To polish it; a piece of wet 



Fig 4 —Optician’s lathe for grinding spherical lenses. 

broadcloth or felt is smoothly applied to the surface of the 
tool upon which the glass was ground, conforming, of 
course, to that surface. The cloth, being sprinkled with 
wet “rouge” (a carefully calcined sulphate of iron), gives 
the glass held against it a beautiful polish without altering 
its spherical curvature. The same processes must now 





























































20 


SPECTACLES AND EYEGLASSES 


be gone over with the other surface of the lens, after which 
it is cleaned and cut to a shape suitable for its future 
mounting. 

This is done by means of an implement called a lens- 
cutter, in which the lens rests on a leather cushion and is 
held firmly in position by a rubber-tipped arm, while a 
diamond-tipped glass-cutter, guided by a pattern, traces 
the oval or other desired outline upon the glass. The 
superfluous glass is removed piecemeal by means of pincers, 
and the lens passes to the next process, which is the 
smoothing and, if necessary, beveling of the edges. This 
is done by hand upon large Scotch grindstones. If the 
lens is to be mounted in a round eye wire, its edge must be 
grooved by means of a file, while a skeleton frame will 
require the drilling of the glass, which may be done by hand 
with a steel drill or by a special machine. 

In grinding a cylindrical lens the surface of the tool is, 
of course, a portion of the surface of a cylinder, and the 
glass is ground by a to-and-fro motion. It is evident that 
the position of the axis of the cylinder in the future spec¬ 
tacle need not be taken into account in grinding, but only 
in the process of cutting to shape for mounting. 

When the lenses are of high power it is of advantage 
that they be made in the form of a meniscus, giving what 
are known as periscopic glasses. For instance, if a + 4. 
diopter lens is required, the anterior surface is ground to a 
+ 6. D. and the posterior surfaces to a — 2. D. It is just 
as advantageous to a cylindrical or sphero-cylindrical 
glass to be periscopic as it is to a spherical, but under 
the previously described methods of grinding it is mani¬ 
festly impossible to give them this form, as the cylinder 
is ground on one side, and the other ground to a plane or 
sphere, as the case may be. 

To make a sphero-cylindrical lens in the form of a 
meniscus it is necessary to grind one surface to the 
toric form. 1 he tore (Latin torus ) is the surface engen- 


GENERAL CONSIDERATIONS 


21 


dered by a circle which turns about an axis situated on the 
plane of the circle. A familiar example of the torus is the 
circular convex molding at the base of an architectural 
column. Fig. 5 shows the tool for rough-grinding a 
convex toric lens. The glass is ground on the inner, 
concave surface of the tool. Fig. 6 is the convex tool for 
grinding a concave tore. A glass ground upon a wheel 
having this form will present two cylindrical curves at 
right angles to each other, one depending on the radius 
of the wheel, and the other on the radius of the convexity 
of its rim. One surface of a lens being made toric the 
other is left for any desired spherical curve. In practice 



Fig. 5- FlG * 6 - 


it is the convex tores which are more often used, combined 
with concave spherical surfaces. A series of blanks is 
generally keptJn stock with the convex toric side ready 
ground. These blanks usually^have a base curve of + 6. 
(Sometimes of +9.) That is, of the two curves of the 
toric surface the lower is T - 6. the other curve is in series, 
beginning at —1— 6.12 and extending, perhaps, to T - 14* 
The difference between the base curve and the other 
represents the cylindrical element in the future lens. 
If it is desired to produce, for example, a tone lens having 
the effect of + 2. Sph. O + 1. Cyl., a blank is selected 
having its toric side ground +6. and +7., or, to state it 
differently, a T~ 1. Cyl. on a T6. base curve. The other 
side of the blank is then ground - 4- sphere. If the effect 
of a simple convex cylinder is required, the concave side of 


22 


SPECTACLES AND EYEGLASSES 


the blank must be ground — 6. That is, equal to the base 
curve. If a minus effect is desired the curve of the concave 
side must be made greater than those of the toric side. 
For example, to produce an effect of — 2. Sph. O — 1. Cyl., 
using the blank before mentioned, it would be necessary 
to grind the concave side — 9. spherical. Blanks are 
furnished, however, having the inner surface ground to 
concave tores on a — 6. base curve. 

Toric lenses, together with deep curved meniscus lenses, 
which are also popularly called torics, have almost dis¬ 
placed the old flat form of lenses in the better class of work. 

Eye Wires, Temples, and Bridges. —Eye wires are made 
by wrapping the untempered wire, in the form of a spiral, 
closely about a metal cylinder. Being tempered while 
in this position, the loops of the spiral will retain the 
shape given them. A single cut down the side of the 
cylinder converts each loop into a separate ring. End 
pieces and straight temples are stamped from sheets of 
metal, and afterward formed and tempered. Hook 
temples of steel are turned from wire upon a lathe. B ridges 
are usually made of oval or half-oval wire, and are simply 
pressed to the desired shape by a forming machine. 

Of the Different Patterns of Spectacles. —In the com¬ 
mon and strongest form of spectacle, the edge of the glass 
is beveled so as to enter a groove in the wire which sur¬ 
rounds it. In a second form, in which the edge of the glass 
is grooved for the reception of a fine, round wire, the object 
sought, of rendering the rim of the spectacles less conspicu¬ 
ous, is generally defeated by the fact that the glass must be 
made thicker than it otherwise need be, in order to give 
room for the groove on its edge. In concave glasses this 
is not the case, since the edge of the glass is here the thick¬ 
est part, and such glasses may sometimes be mounted in 
this way with advantage. In a third form, called a rim¬ 
less’ J spectacles (Fig. 7), the wire encircling the glass is 
dispensed with altogether, small holes being drilled through 


GENERAL CONSIDERATIONS 


23 


the glass near its edge for the accommodation of screws 
which fasten the bridge and temples in place. The advan¬ 
tages of this form are its beauty and inconspicuousness. 



It should never be prescribed for children, as it is quite 
liable to break at the point where the glass has been drilled. 
The edges of these glasses should not be polished, but 



should be given a dull finish, otherwise they reflect the 

light disagreeably. . . 

Sides, or temples, have been variously constructed. 

Those having sliding and turn-pin joints are examples of 











24 


SPECTACLES AND EYEGLASSES 


antiquated forms. Those now used are the “hook,” or 
“riding-bow,” the “half-hook” and the plain, “straight” 
temple. The former are to be preferred in all cases where 
the glasses are to be worn constantly or nearly so, and the 
latter for those who wear glasses for near work only, and 
require to remove them frequently from the eyes. Hook 
temples are made in three lengths, 5^ inches, 6 inches, and 
inches, designated as short, medium, and long. These 
are sufficient for all cases. The half-hook is suitable for 
heavier temples and is used a good deal in celluloid frames. 

By far the most useful spectacle bridge is the well known 
saddle bridge shown in Fig. 8. If properly made and of a 
sufficient length of wire it gives the fitter entire control of 
the position of the lenses in any given case and admits of 
nice adjustments in all its dimensions. The “K” bridge, 
formed of wires in the shape of the letter K, is allowable 
in some cases. The nearly similar “X” bridge allows the 
glasses to teeter, or see-saw across the nose, with the 
motions of the head. It is, however, the best form of 
bridge for reversible glasses; that is, glasses for persons hav¬ 
ing sight in one eye only, who may have their distant glass 
set in one side of a frame and their near glass in the other. 
By using this bridge and straight temples, or hook temples 
without a shoulder at the hinge, the spectacles may be 
turned over so as to bring either lens before the wearer’s 
seeing eye. The old-fashioned bridge, called the “curl,” 
is unobjectionable for cases in which the bridge of the nose 
is prominent, or for the spectacles of old people, who like 
to slip their glasses down toward the end of the nose. A 
small piece of cork is sometimes attached to the under 
side of the bridge where it comes in contact with the skin. 
It is unnecessary if the frames fit the face of the wearer 
properly. If it be desirable to remove all pressure from 
the bridge of the nose and transfer it to the sides, it may be 
done by soldering a pair of guards, similar to those used 
on eyeglasses, to the spectacle bridge. A much better 


GENERAL CONSIDERATIONS 


25 


way, however, is to use a special spectacle bridge of which 
there are several forms. One of the best of these is shown 
in Fig. 9. Its excellence consists in the shape of the wire 
to which the nose guards are soldered. It is such as to 
allow a wide range of adjustment. 

The earliest spectacles appear to have had round eyes 
and late years have seen a revival of this shape in sizes 
ranging from 38 to 42 mm. diameter. It now disputes 
with the oval the first place in popular esteem. It is 



certainly the most convenient shape of any and is, perhaps, 
as generally becoming as any other. 

Table II.— Names and Sizes oe Standard Ovals 

No. 000. 41 by 32 mm. = Round 36.8 mm. Diameter. 

00. 40 by 31 mm. = Round 35.6 mm. Diameter, 

o. 38.5 by 29.5 mm. = Round 33.5 mm. Diameter. 

1. 37 by 28 mm. = Round 32.5 mm. Diameter. 

The old standard oval eye has a difference of 9 mm. in 
its major and minor diameters. The vogue of large 
lenses, and of round lenses, has made the old ovals look 
antiquated and has led to the extensive use of shorter, 
broader ovals based on a difference of 5, 6, 7, or other 
number of mm. in the diameters. Each of these shapes, 




26 


SPECTACLES AND EYEGLASSES 


of course, has its own series of sizes. Table II shows the 
numbers and dimensions of the old standard ovals and also 
the diameter of the round lenses having equivalent areas. 
The larger sizes are known as 38, 40, 42 and 44 mm. ovals. 
Taking the 40 mm. oval for example, the name indicates 
that it will fit the same eye wire as the 40 mm. round lens. 



Pig. 10. —Shapes of ovals and drop-ovals. 


It does not indicate the length or breadth of the oval, 
since these vary, depending on whether the oval is cut 
with 6, 7, 8, or 9 mm. of difference between its major and 
minor diameters. 

The “drop-oval” or “student” shape is one in which 
breadth is added to an oval below its horizontal diam¬ 
eter with the idea of better suiting the glass to overhanging 
brows or to persons who complain of seeing beneath their 


GENERAL CONSIDERATIONS 


27 


glasses. Sizes of these lenses may be named, like the 
true ovals, from the diameter of the round glass of equal 
circumference. 

The octagon is an old shape which probably had its 
origin in the difficulty of cutting and mounting true ovals 
at an early stage of the optician’s art, and the demand for a 
narrow shape for presbyopes. The quest of novelty has 
led to an attempt to revive this shape. 

Where glasses are used for near work only, the eyes are 
sometimes made of semi-oval shape, allowing the line of 
sight to pass over their upper, straight edge when the 



C 





Fig. 11. 


wearer views a distant object. These are known as “half” 
“pulpit,” or “clerical” eyes. The increased use of 
bifocal lenses has almost driven them out of use. 

Bifocal Glasses.—When glasses of different focusing 
power are required for distant and near vision, the trouble 
incident to frequent changing is obviated by “bifocal” 
glasses. That is, the lower part of the spectacle eye, which 
is used for near work, is made to differ in focusing power 
from the upper part, which is used for distant vision. Such 
bifocal glasses are also called Franklin glasses, from the 
philosopher who, as we have seen, invented them. 









28 


SPECTACLES AND EYEGLASSES 


The object sought may be attained in various ways. In 
the early Franklin glasses each eye contained two half-oval 
pieces, with their straight edges in apposition (A, Fig. n). 
This has been improved upon by making the line of junc¬ 
tion a curved one, giving somewhat greater latitude of 
distant vision and rendering the glass more secure in its 
frame. Both these lorms are now obsolete. A more 
successful form of the two piece bifocal is the cemented 
bifocal” shown at D, Fig. 11. To the back or front surface 
of the distance glass is cemented, by means of Canada 
balsam, a small lens whose strength, added to that of the 




Fig. 12. 




distance glass, equals the glass required for near work. 
The upper edge of the supplemental lens should be ground 
as thin as possible in order to render it inconspicuous. 
A special grade of these lenses is made by a patented process 
by which the supplemental lens is ground very thin by 
fastening it to a block of glass instead of one of iron, and 
grinding the two pieces of glass away together. These 
spectacles are strong, light and handsome, and may 
readily be made in the rimless form. For cylindrical 
lenses this arrangement is, moreover, cheaper than the 
others, since only the distance glass need have the cylinder 
ground upon it, the supplemental segment being a simple 






GENERAL CONSIDERATIONS 


29 


sphere. The changes in the correction for near which are 
likely to be needed from time to time are readily and 
cheaply effected in this form of bifocal glass. 

The shape and size of the supplemental segment may be 
varied to suit all exigencies of use or taste. Fig. 12 illus¬ 
trates some of the more common forms, of which B and C 
are the most useful. 

The cemented bifocal lens still holds a place by reason 
of its cheapness and its ready adaptability to different 
cases and conditions. Its inherent weakness is the cement 
which is apt to become soft or brittle or opaque under the 
exigencies of use. 

In still another form of bifocal glass, which did not go 
beyond the experimental stage, the small supplemental 
lens figured at D is made of flint glass of high refractive 
index and is countersunk, that is to say, is cemented into a 
corresponding concavity ground in the distance glass. 

“Kryptok,” or fused bifocals, are a further evolution of 
the countersunk supplemental lens. In their manufacture 
a small lens of flint glass is let into a large lens of crown 
glass by countersinking, as described above. Instead of 
cementing the supplemental lens in position, however, the 
lenses are heated to the point of fusion of the glass, when 
its two portions unite. The surfaces of the glass are then 
ground. One surface of the small supplemental lens is 
exposed to the grinding and is reduced to the same curva¬ 
ture as the corresponding surface of the main lens. The 
necessary difference in the refraction of the upper and 
lower portions is dependent on the difference in index of 
refraction of the crown glass of which the main lens is 
composed and the flint glass of the supplemental lens. 

This lens is made in both flat and meniscus form. 
The small supplemental lens is placed on the side away 
from the eye. In the meniscus formed lenses the optician 
keeps in stock “blanks” having the bifocal (convex) side 
ground and finished to some convenient spherical curve, 


30 


SPECTACLES AND EYEGLASSES 


4 - 4., + 6., + 8., or +10. A concave sphere or concave 
tore is then ground on the inner face of the glass. 

“Ultex” bifocal glasses are made in one piece by a 
special method of grinding which produces two different 
spherical surfaces on one side of the lens. This is an 
old idea, and the old form of the glass is shown at C, Fig. 
11. This old glass is very faulty from a prismatic effect 
inherent in the method of manufacture, and never came 
into general use. 

The ultex glass is made in meniscus form only. The 
bifocal surface is the posterior or concave one. “Blanks” 



Fig. 13. 


have a base curve of — 4., — 6., — 8., or — 9. The lower 
part of the posterior surface, however, is ground on a 
separate tool to some less degree of concavity. The 
difference in curvature of the two portions of this concave 
surface represents the reading addition in the finished lens. 
The blanks are furnished in series with reading additions 
from .50 D. to 4.50 D. 

To produce any given ultex bifocal the optician selects 
a blank having a suitable base curve with the proper 
reading addition and grinds on the front surface the 
required convex spherical curve. Where a cylindrical 
element is desired the front surface is given a convex 
toric form. For example, to produce an ultex glass + 1. 
sphere with + 2. sphere added for reading, a blank is 
selected whose base curve is — 6. and which has a + 2. 


GENERAL CONSIDERATIONS 


3 1 


reading addition. (That is, its reading portion has a - 
4. curve.) The front of this blank is then ground T 7 * 

To make a glass + 1. sphere O + 1. Cyl., with+ 2. 
sphere added for reading, the same blank would be used as 
in the last instance but the front surface would be made 

convex toric with its curves + 7 - and + 8. 

Both theoretically and practically the ultex lens repre¬ 
sents the highest development of the bifocal idea. The 
fused glass is not so perfect optically or mechanically. 
When strong reading additions are made in this form the 
main lens is deeply countersunk and must be made 
correspondingly thick and heavy. In the process of fus¬ 
ing the two lenses together there is unavoidably some 
loss of definition at the joined surfaces. A more notice¬ 
able fault is the color aberration seen in some combina¬ 
tions, consequent on joining a convex flint lens of high 
color dispersion value to a concave crown lens of low 
dispersion. The ultex is free from these sources of trouble. 
It may be made as light as desired and objects seen 
through its reading portion are free from color aberration 

and with unimpaired definition. 

Since the one-piece bifocal has demonstrated its supe- 
riority to all other forms, lenses have been produced which 
avoid the use of the special machinery by which the 
ultex lenses are ground and polished. In these, the surface of 
the reading portion is sunk below the level of the surface 
of the main lens. There is thus a shoulder or inset at the 
junction. These lenses compete only with the cement 
bifocal, than which they are more durable but also more 

expensive. 

Some persons declare that they cannot become accus¬ 
tomed to bifocals however well adjusted. Parallel, hori¬ 
zontal lines, as those of a staircase, are particularly 
confusing, it being possible to see each line doubled 1 . e 
junction of the two segments of the glass is placed jus 
opposite the pupil. Such persons may prefer having 


32 


SPECTACLES AND EYEGLASSES 


an u extra front” (Fig. 14): that is, a second pair of spectacles 
whose temples are replaced by short hooks, by means of 
which they are hung in front of the frame already upon 
the face. This is a rather clumsy device; less so, however, 
when the eyes of the extra front are made half oval instead 
of oval. 

Eyeglasses.—The increased popularity of eyeglasses 
has stimulated invention and they now present a greater 
variety of forms than do spectacles. Every part is sub¬ 
ject to such change and adjustment that it is quite possible 
to fit them to cases which were out of the question with 
the old forms. 



Pig. 14. 


An eyeglass is held in place by the pressure of a spring, 
or springs, upon the sides of the nose. There are four 
methods of placing this spring and hence four radically 
different forms of eyeglasses. First, we have the old and 
familiar form of arched spring (Figs. 15 and 16) which allows 
the frame to “open” while the lenses remain in their orig¬ 
inal plane. Second, the spring may be placed in a plane 
approximately at right angles to that of the lenses and the 
frame opens by the outer ends of the lenses moving forward. 
(Fig. 17.) Third, the lenses are joined by two bars 
sliding over each other in connection with a spiral spring. 
The frame opens when the two lenses are drawn apart 
along the line of their long axes. (Fig. 18.) Fourth, 
the lenses are joined by a rigid bridge, like a spectacle, 
while the nose-pieces are directly connected with spiral 
springs which press them to the sides of the nose. To 
open the frame special levers for the thumb and finger 






GENERAL CONSIDERATIONS 


33 


are provided by means of which the nose-pieces are pressed 
apart. (Fig. 19.) In another variety these levers are 
operated by pressing forward the outer ends of the lenses. 






Fig. 18.—Rigid frame or “bar spring” eyeglasses. 


The first of these forms still holds its place as the most 
generally useful. It is simple and inconspicuous and the 
spring acts directly. For the majority of cases it is to be 

3 











34 


SPECTACLES AND EYEGLASSES 


preferred. Its spring may be made light or heavy and may 
be offset from the plane of the lenses to accommodate over¬ 
hanging brows. The second form offers no marked superi¬ 
ority over the first, from which it differs only in the 
direction of the action of the spring. This may make its 
hold more certain in a few cases, and occasionally persons 
who have tried both will prefer this form. The third and 
fourth forms have the same object in view, namely, to 
hold the lenses in position with the certainty of a spectacle 
frame, allowing none of that displacement of the axis of a 
cylinder, or base of a prism which is possible when they are 
joined by a yielding spring. The form designated in this 
description as 11 third,” and which is usually called the 



Fig. 19 


“bar-spring” eyeglass and of which there are several 
variations (Fig. 18) has not met with much favor. They 
are heavy and cumbersome looking and there is apt to be 
lost motion between their sliding bars, which allows the 
displacement they are intended to prevent. The last of 
our four forms has more merit. Here, the junction 
between the lenses is really rigid, and no alteration of their 
relation to each other can take place. It is possible, how¬ 
ever, for one lens to be displaced upward and the other 
downward, or for one to stand forward and the other back¬ 
ward. This frame contains two springs, instead of one as 
in the other forms, and weakening of one spring may cause 
one of these displacements. Moreover, the small spiral 
springs are not very durable. In spite of its limitations, 




GENERAL CONSIDERATIONS 


35 


however, this frame has distinct value in some cases, 
especially where the lenses are heavy. 

Nose-pieces are furnished in many forms. The use of 
the old form (Fig. 2) which lies in the same plane as the 
lenses, is now quite limited. The “ offset guard/’ which 
bears upon the nose posterior to the plane of the lenses is 
much more generally useful. The latter should certainly 
be preferred for any case in which the glasses are to be worn 
for more than a few minutes at a time. There are dozens 
of varieties of the offset guard, many of which exist only 
for trade purposes and are advertised much beyond any 



A. For shallow bridge, prominent eyes, flat forehead. 

B. For shallow bridge, prominent eyes and forehead. 

C. The guard used for the average case. 

D. Deep-set eyes, prominent nose and forehead. 

E. Same as C, but for lowering glasses (for reading). 

F. Same as B, but for lowering glasses (for reading). 

G. Same as C, but somewhat smaller and neater, although having less 
bearing surface. 


peculiar merit which they possess. Usually, the fewer 
and simpler the parts of any implement the better. The 
best guard is one stamped from a single piece of metal. 
Rivets will frequently loosen or fall out, or the metal may 
split to a rivet hole. As for pivots in nose-pieces, they are 
a delusion. What we should seek is not a self-adjusting 
eyeglass, but one capable of wide adjustment, and which 
will keep the shape which we give it. To this end the 
metal should be tough and pliable, though possessed of a 
certain amount of rigidity. The bearing surfaces of 
nose-pieces may be covered with cork, shell or celluloid. 
They soon become greasy and slippery, and beside being 
thick and clumsy, are not durable. If one tries to mold 






















SPECTACLES AND EYEGLASSES 



such nose-pieces to a different shape, the cork or shell 
frequently breaks, or the rivets pull out. The later pat¬ 
terns are stamped from a single sheet of metal, without 
any covering for the bearing surfaces, which are corru¬ 
gated, fenestrated, or divided into two or more portions to 
give a more clinging hold upon the skin. In adjusting 
these nose-pieces to the patient’s face they may be shaped 
with the pliers with much greater freedom than can the 
other forms. 

The “arm’’ or “foot” of the nose-piece is, in most forms, 
made in several lengths and shapes, for use on variously 





Offset Styles (3 mm. off center) 




CO DO 


> 4 

Fig. 21. 




proportioned faces (Fig. 20). Much of the adjusting of 
eyeglasses is done by bending and twisting this arm. 

Studs of eyeglasses are made in about six different 
lengths, from 1 mm. to 6 mm. as shown in Fig. 21. By 
means of these the intercentral distance of the lenses may 
be varied. In each of these lengths an offset stud is made, 
which may be used to place the lenses farther forward. 
Moreover, there are angular forms useful in tilting the 
lenses. 

Spectacles for Cosmetic Effect. —Something may legiti¬ 
mately be done, at times, in the way of improving the 
appearance of a patient by the application of glasses. The 




GENERAL CONSIDERATIONS 


37 


blind whose eyes are not only sightless, but unsightly, very 
commonly hide them behind colored glasses. Neatly fit¬ 
ting spectacles with large eyes of ground glass render the 
appearance of such persons less lugubrious. When one 
eye is useless for vision, and at the same time small, and 
the orbit undeveloped, a gratifying improvement in the 
appearance of the patient may be attained by placing before 
the shrunken eye a convex glass of sufficient strength to 
magnify it to the size of its fellow. The condition known 
as epicanthus can generally be removed by wearing eye¬ 
glasses whose nose-pieces draw just enough on the inner 
canthi to smooth out the offending fold of skin. As the 
subjects of epicanthus are generally flat-nosed, it may be 
necessary to furnish the eyeglasses with a pair of hook 
temples to keep them in place. Since operations for this 
disfigurement are so unsatisfactory, such an appliance is 
probably the best treatment we can advise in case the 
trouble is not outgrown. 


II. THE PRINCIPLES OF SPECTACLE 

FITTING 


We have now to consider the essential principles of plac¬ 
ing glasses before the eyes. The usefulness of spectacles 
depends almost as much upon the fidelity with which these 
principles are carried out as it does upon a careful correc¬ 
tion of the errors of refraction. 

Centering and Decentering. —By the visual axis, or, in 
English, the line of sight, is meant a line from the yellow 
spot of the retina through the nodal point of the eye to the 
object sighted. 

By the principal axis of a lens we mean a line passing 
through the optical center of the lens (the thickest part, if 
the lens is convex; the thinnest if concave) at right angles 
to its surfaces. 

The geometrical center of a spectacle glass may be 
shortly said to be that point on its surface which is equally 
distant from the extremities of the figure to which it is cut. 
The principal axis of the lens may or may not pass through 
this latter center. 

We habitually regard as the normal position for glasses 
one in which, when the eyes are looking at a distant object, 
the visual axes correspond exactly in position with the 
principal axes of the lenses, and together they pass through 
the geometrical centers of the spectacles. In other 
words, the geometrical center of the spectacle eye and the 
optical center of the spectacle lens coincide, and the 
center of the pupil for each eye lies directly behind them. 
Regarding decentering, some confusion is apt to arise 
because the word is used in two different connections. 
If the visual axis pass to the temporal side of the optical 

38 


THE PRINCIPLES OF SPECTACLE FITTING 


39 


center of a glass held before an eye, then, with respect to 
that eye, the glass is said to be “decentered in.” If the 
visual axis pass to the nasal side of the optical center of the 
glass, the latter is “decentered out.” Similarly a glass 
may be decentered in any other direction. When speaking 
of spectacles, however, without reference to the eyes of the 
wearer, they are said to be “decentered in” when their 
optical centers lie to the inner side of their geometrical 
centers; “decentered out” when the optical centers are 
to the external side of the geometrical centers, etc. A 
glance at Fig. 22, which represents a pair of spectacles 
decentered in, will make clear what is meant. 



G G show the position of the geometrical centers; O O, that of the optical 

centers. 

From the above it will readily be seen that when it is 
desired that a patient wear decentered lenses, the effect 
may be obtained in either of the two ways; first, by decen¬ 
tering the lenses in their frame; second, by displacing 
them, together with their frames, from what I have 
described as the normal position. The first method has 
the disadvantage of increasing the weight of the glass, 
while the second limits the field of binocular vision. In 
practice, the second method should be employed to the 
greatest extent possible without unduly interfering with 
binocular vision for the distance at which the spectacles 
will be used, and, should still farther decentering be 
required, the method first mentioned should be brought 
into service. For instance, suppose we wish to order 
glasses with each lens decentered in 8 mm. This would 
mean that the optical centers are to be 16 mm. nearer 





40 


SPECTACLES AND EYEGLASSES 


together than the patient’s pupils. Let us suppose that by 
a careful consideration of the distance for which the glasses 
are prescribed, of the distance at which they must be 
placed in front of the eyes, and of the size of the spectacle 
eye used, we find that the frame can be made only io mm. 
narrower than normal without the outer rims of the “eyes” 
becoming annoying. This leaves 6 mm. to be obtained by 
decentering the glasses in their eye wires. If the distance 
between the patient’s pupils were 60 mm., we would order 
the distance between the geometrical centers of the spec- 



Fig. 23. Fig. 24. 

Figs. 23 and 24. —Showing the prismatic effect of decentering. 

The optical center, O, in Fig. 23 coincides with the geometrical center 
G. In Fig. 24, which represents a decentered lens of the same spherical 
curvature, O has been removed toward the base of the virtual prism b a c. 
{After Maddox .) 

tacle eyes to be 50 mm., and each eye to be decentered in 

3 mm. 

Prismatic Effect of Decentering. —It is to obtain a pris¬ 
matic effect from spherical lenses that decentering is gener¬ 
ally ordered, since a decentered lens is identical with a 
lens of the same strength combined with a prism. This 
is graphically shown by Figs. 23 and 24, the latter of which 
represents a section of a decentered lens, which will readily 
be seen to be precisely the same as the result would be 
if the normally centered lens shown in Fig. 23 were split 
into halves and the prism b a c introduced between them. 

The size of the glass disk from which spectacle lenses are 
ground will not allow of more than about 2 mm. of lateral 
decentering for a No. 1 eye; 3 mm. for Nos. 2 and 3; and 

4 mm. for No. 4. Vertically, they may be decentered 
much more. When ordered to decenter laterally more 











THE PRINCIPLES OF SPECTACLE FITTING 


41 


than this, or to furnish a prismatic effect greater than 
can be obtained by this much decentering, the optician 
first manufactures a prism of the requisite strength, and 
then grinds spherical surfaces upon its two faces. It is, 
therefore, of not much importance whether, in ordering a 
sphero-prismatic combination, we express the prismatic 
element in degrees of the refracting angle, or in millimeters 
of decentration of the lens: the optician produces the glass 
by whichever method is the more convenient. 

The stronger the lens, the less decentering it requires to 
produce a given prismatic effect, and where the combina¬ 
tion desired is that of a strong lens with a weak prism, the 
more accurate practice probably is to order the lens 
decentered the requisite number of millimeters. For this 
purpose a table of equivalents, such as is given below, 


Table III.* —Decentering Equivalent to a Given Refracting Angle 

(Index of Refraction, 1.54) 


Lens 

i° 

2° 

3° 

4 

i D, 9.4 

l8.8 

28.3 

37-7 

2 

4-7 

9-4 

14.1 

18.8 

3 

3-i 

6-3 

9-4 

12.6 

4 

2-3 

4-7 

7 -i 

9-4 

5 

1.9 

3-8 

5-7 

7-5 

6 

1.6 

3 • 1 

4-7 

6-3 

7 

i -3 

2.7 

4 

5-4 

8 

1.2 

2.3 

3-5 

4-7 

9 

1 

2.1 

3 • 1 

4.2 

10 

•9 

1.9 

2.8 

3-8 

11 

•9 

i -7 

2.6 

3-5 

12 

.8 

1.6 

2.4 

3 -i 

13 

•7 

1 -4 

2.2 

2.9 

14 

• 7 

i -3 

2 

2-7 

i5 

.6 

1 -3 

1.9 

2-5 

16 

.6 

1.2 

1.8 

2.4 

17 

.6 

1.1 

i -7 

2.2 

18 

•5 

1 

1.6 

2.1 

19 

•5 

1 

i -5 

2 

20 

• 5 

•9 

1 -4 

i -9 


5° 

6° 

8° 

10° 

47.2 

56.5 

75-8 

95-2 

23.6 

28.2 

37-9 

47.6 

15-7 

M 

CO 

00 

25-3 

3i -7 

11.8 

14.1 

18.9 

23.8 

9-4 

11 -3 

15-2 

19 

7-9 

9-4 

12.6 

15-9 

6.7 

8.1 

10.8 

13-5 

5-9 

7-i 

9-5 

11.9 

5-2 

6-3 

8.4 

10.5 

4-7 

5-6 

7.6 

9-5 

4-3 

5-i 

6.9 

8.7 

3-9 

4-7 

6-3 

7-9 

3-6 

4-3 

5-8 

7-3 

3-4 

4 

5-4 

6.8 

3-i 

3-8 

5-i 

6-3 

3 

3-5 

4-7 

6 

2.8 

3-4 

4-5 

5-6 

2.6 

3-i 

4.2 

5-3 

2-5 

3 

4 

5 

2.4 

2.8 

3-8 

4.8 


* Jackson: 
1889. 


“Transactions of the American Ophthalmological Society.” 








42 


SPECTACLES AND EYEGLASSES 


is necessary. To use it we find in the first column the 
strength of the lens used, and on a level with this, in the 
column at whose head stands the strength of the prism 
required, is given in millimeters the amount of decentration 
necessary. 

It is one of the beauties of the reformed numbering of 
prisms (see page 70), that by a simple calculation one can 
tell in a moment the amount of decentration required to 
produce any required number of centrads, by means of any 
given lens. 

Divide the number of centrads required by the strength 
of the lens, in diopters. The quotient is the necessary 
decentration, in centimeters . For example: to produce a 
prismatic effect of 3. Cr. by means of a lens of 5. D., it is 
necessary to decenter as many centimeters as 5 is contained 
times in 3, which is .6 centimeters. 

Table IV is constructed by applying this rule. In it, 


Table IV. —Decentering Equivalent to a Given Number of 

Centrads 


Lens 

1 Cr. 

2 Cr. 

3 Cr. 

4 Cr. 

5 Cr. 

6 Cr. 

8 Cr. 

10 Cr. 

iD> 

10 

20 

30 

40 

50 

60 

80 

100 

2 

5 

10 

15 

20 

25 

30 

40 

50 

3 

3-3 

6.6 

10 

13-3 

16.6 

20 

26.6 

33-3 

4 

2-5 

5 

7-5 

10 

12.2 

15 

20 

25 

5 

2 

4 

6 

8 

10 

12 

16 

20 

6 

1.6 

3-3 

5 

6.6 

8.3 

10 

13-3 

16.6 

■ 7 

1.4 

2.8 

4.2 

5-7 

7 -i 

8.2 

11.4 

14.2 

8 

1.2 

2-5 

3-7 

5 

6.2 

7-5 

10 

12.5 

9 

1.1 

2.2 

3-3 

4.4 

55 

6.6 

8.8 

11.1 

10 

1 

2 

3 

4 

5 

6 

8 

10 

11 

•9 

1.9 

2.8 

3-7 

4.6 

5-5 

7-3 

9 

12 

.8 

1.8 

2-5 

3-3 

4 -i 

5 - 

6.6 

8-3 

13 

•7 

i -5 

2.3 

3 

3-8 

4.6 

6.1 

7.6 

14 

•7 

1.4 

2.1 

2.8 

3-5 

4.2 

5-7 

7 -i 

15 

. 6 

i -3 

2 

2.6 

3-3 

4 

5-3 

6.6 

16 

.6 

1.2 

1.8 

2-3 

3-i 

3-7 

5 

6.2 

1 7 

•5 

1.1 

i -7 

2-3 

2.9 

3-5 

4-7 

5.8 

18 

•5 

1.1 

1.6 

2.2 

2.7 

3-3 

4.4 

5-5 

19 

•5 

1 

i -5 

2.1 

2.6 

3 -i 

4.2 

5-2 

0 

•5 

1 

i -5 

2 

2-5 

3 

4 

5 




THE PRINCIPLES OF SPECTACLE FITTING 


43 


however, the distances which the lenses must be decentered 
have been reduced to millimeters by moving the decimal 
point one place to the right, in order to make it practically 
more convenient, and render it homologous to Table III, 
like which it is used. 

A cylindrical lens, or the cylindrical element of a sphero¬ 
cylindrical lens, when decentered in a direction vertical to 
its axis, acts as a spherical lens of the same strength. 
Thus, a + 2.Sph. O + i.Cyl. axis vertical, decentered 
horizontally, would have the same prismatic effect 
as a + 3-Sph. treated in the same way. As the axis is 
inclined toward the direction of decentration, the prismatic 
effect of the cylinder diminishes, and disappears when they 
coincide. Thus, a + 2. Sph.O + 1. Cyl axis horizontal, 
decentered horizontally, would have merely the prismatic 
effect of a + 2. Sph. so treated. 

Normal Lateral Centering. —In proportion as the pris¬ 
matic effect of decentered lenses is a valuable property 
where this effect is desired, it has to be guarded against in 
those cases which do not require it, to which number 
belong, of course, the great majority of the cases we are 
called upon to treat. If the objects looked at through 
spectacles were always situated in the same direction and at 
the same distance, fixing the position proper for the centers 
would be a simple matter; but, in the movements of the 
eyes, each pupil roves over a territory some 18 mm. (% in.) 
long by 15 mm. broad. When the eyes are directed 
toward a distant object the centers of the pupils are about 
60 mm. apart, and on convergence only 56 mm. so that the 
proper adjustment of spectacles is a series of compromises 
between that proper for the position of the eyes in which 
the glasses will be most used and other positions in which 
they will be less used. Of course, the position in which they 
will be most used must receive the greatest consideration. 

The proper position for the centers of distance 
glasses has already been stated. When glasses are to be 


44 


SPECTACLES AND EYEGLASSES 


used for near work only, they should be decentered “in” 
two or three millimeters on each side from this “normal” 
position, as such glasses, being never used in that position, 
but only when the visual axes are converged, would other¬ 
wise never be rightly centered. What amounts to the 
same thing, and is more often done, is to make the front 
of the near spectacles four or six millimeters narrower than 
if they were intended for distant vision: four millimeters 
narrower for a working point of 15 inches; six millimeters 
narrower for one of 10 inches. Concerning the centering of 
glasses which are worn constantly, no rule for all cases can 
be laid down, since accurately centering for any one dis¬ 
tance is decentering for every other. Fortunately, as a 
glance at Table III will show, it is only with lenses of high 
power that a considerable amount of prismatic effect is 
developed by slight decentering. Where such glasses 
must be worn constantly by a person who spends several 
hours daily at near work, they should certainly be slightly 
decentered inward. 

The distance between the geometrical centers is regulated 
by the size of the spectacle eyes and the width of the space 
between them occupied by the bridge. Where the inter¬ 
pupillary distance is short, as in children, opticians are apt 
to make the eyes of the spectacles so small as to interfere 
seriously with the field of vision through them. With the 
saddle-bridge there is no difficulty in diminishing the space 
between the spectacle eyes without interfering with the 
form of that part of the bridge which is applied to the 
nose, and the required adjustment should be made in this 
way, leaving the spectacle eyes of good size. 

Normal Vertical Centering. —The glasses require, fur- 

' y. 

ther, to be so placed that the points where the wearer’s 
visual axes penetrate them shall neither be above nor below 
the centers. This adjustment is readily seen to depend 
upon the relative height of the bridge of the spectacles and 
the bridge of the nose at the point where the spectacles 


THE PRINCIPLES OF SPECTACLE FITTING 


45 


rest. The higher the spectacle bridge, the lower will the 
glasses stand upon the patient’s face, and vice versa. 

On the bridge of nearly every nose there may be felt a 
point at which the narrow, upper portion of the nasal bones 
gives place rather suddenly to the broader lower portion. 
Just here, in what has been called the “natural” position 
(A, Fig. 25), the bridge of the spectacles tends to rest, and 
the attempt to make it remain at any other point will not 
be very successful. In distance spectacles, then, the 
bridge should be made of such height that when resting at 
this natural position, the centers of the spectacle eyes are 



Fig. 25. 


at the same height as the centers of the pupils when the 
patient looks straight forward. When the glasses are to be 
used for near work only, their bridge should be made about 
2 mm., or inch, higher than otherwise, allowing the 
centers to drop that much lower, as the wearer’s eyes will 
nearly always be directed to objects below their own level. 

Distance of the Glasses from the Eyes— As a rule, the 
glasses should be placed just far enough from the eyes, 
to escape the lashes in the act of winking. If the lashes 
touch the glass the latter quickly becomes soiled, and to 
the spectacles is, moreover, attributed any falling out of 
the lashes which may occur. Some persons, however, with 
myopia of high degree, prefer the glasses to be placed as 


46 


SPECTACLES AND EYEGLASSES 


close to the eyes as possible, regardless of the lashes, 
because of the larger clear images which they thus obtain. 
This adjustment of the glasses depends upon the relation of 
the top of the spectacle bridge to the plane of the glasses. 
Where the eyes are deep set, or the nose of the aquiline 
type, the top of the spectacle bridge must be in front of the 
plane of the glasses, or, as it is shortly called, “out” 
(Fig. 26). When the bridge of the nose is low and the 
eyes relatively prominent, as in the negro, Chinese, and 
children, the top of the bridge must be back of the plane 
of the glasses, or “in,” as represented in Fig. 27. 


1 

•V •. 


Fig. 27. 

Perpendicularity of the Plane of the Lenses to the Vis¬ 
ual Axis.— A very important requirement is that the 
plane of the correcting lens when in use shall be as nearly 
as possible perpendicular to the visual axis. The stronger 
the lens the more important in this detail, whose warrant 
lies in the fact that the refractive value of a given lens 
placed obliquely to the visual axis is no longer that indi¬ 
cated by its number, but is that of some other, stronger 
lens. A cylindrical lens so placed acts simply as a stronger 
cylindrical lens, a spherical lens; however, as a stronger 
spherical lens combined with a cylindrical lens with its 
axis at right angles to that about which the lens is rotated. 

The results of the investigations of himself and others, of 
the effect of the obliquity of a lens to an incident pencil of 
rays, was summarized by Dr. Edward Jackson in a paper 
read before the American Medical Association in 1877, and 










THE PRINCIPLES OF SPECTACLE FITTING 


47 


their practical application to this part of our subject 
pointed out. From that communication the following 
table is extracted. It gives in the first column the degrees 


Table V 


Obliquity of 
the Lens 

Refractive Power of a 1. D. 
Cylindrical Lens So Placed 

Sphero-Cylindrical Equiva¬ 
lent of a 1. D. Spherical Lens 
So Placed 

o° 

1. D. cyl. 

1. D. spherical. 

5 ° 

1.01 D. cyl. 

1.00 sph.O0.01 cyl. 

IO° 

1.04 D. cyl. 

1 .or sph.O0.03 cyl. 

15 ° 

1.10 D. cyl. 

1.02 sph.00.08 cyl. 

20° 

1.17 D. cyl. 

1.04 sph.Oo. 13 cyl. 

25 ° 

1.30 D. cyl. 

1.06 sph.Oo.24 cyl. 

Oo 

O 

0 

1.44 D. cyl. 

1.09 sph.Oo.36 cyl. 

35 ° 

1.69 D. cyl. 

1.12 sph.Oo.56 cyl. 

0 

O 

2.01 D. cyl. 

1.16 sph.Oo.83 cyl 

45 ° 

2.46 D. cyl. 

1.22 sph.Oi. 24 cyl. 


of obliquity at intervals of 5 0 up to 45°. In the second col¬ 
umn is shown the refractive value of a 1. D. cylindrical, in 
the third that of a 1. D. spherical lens so inclined. 




To fulfil this requirement of perpendicularity to the 
visual axis, the lenses of spectacles used only for distance 
should lie in a vertical plane; that is, they should face 
















48 SPECTACLES AND EYEGLASSES 

directly forward, as shown in Fig. 28. Since the visual 
axes are directed downward and forw r ard when near work is 
done below the level of the eyes, glasses for near must 
face downward and forward, as in Fig. 29, in order that 
the plane in which they lie shall be perpendicular to those 
axes. Furthermore, in viewing near objects the visual 
axes are directed inward and toward each other. 1 his will 
require the glasses to face inward also, as represented in 
Fig. 30, so that they come to lie in different planes, instead 

of in the same plane, as formerly. 

When “ constant” glasses are prescribed, the lenses 
should be placed midway between the proper facing for 



near and that for distance glasses. Then, though the lens 
is not exactly properly inclined either for distant vision or 
near work, the result of such slight obliquity to the visual 
axis is unimportant, since, as a reference to Table V will 
show, it is only in the higher degrees of obliquity that the 
increase in power, and especially the development of cylin¬ 
drical effect from spherical lenses, is rapid. Moreover, by 
slightly bending the neck a moderate degree of obliquity 
of the glasses to the visual axis may be removed without 
discomfort to the wearer. 

The position of bifocal glasses should also be between 
that proper for near and for distance glasses, but nearer 
that of the stronger glass. This will generally be the near 
glass, as convex bifocals are much more frequently pre¬ 
scribed than concaves, and such glasses should face only a 
little less downward than glasses intended entirely for near 


THE PRINCIPLES OF SPECTACLE FITTING « 


49 


work. When concave bifocals are worn, however, they 
should face more forward and much less downward. 

The angle which the plane of the glasses makes with the 
plane of the wearer’s face depends entirely upon the angle 
formed by the plane of the glasses and the temples of their 
containing frames. Thus, when the temples are perpen¬ 
dicular to the plane of the glasses, as in Fig. 28, the latter 
will face forward and not at all downward. They may be 
made to face downward to any required degree by simply 
turning down the temples at the points where they are 
hinged to the end pieces. These must be equally turned 
down, however, as where only one is turned down, or one 
more so than its fellow, the result is not to make the glasses 
face downward, but to make the glass on the side of the 
lower temple ride higher on the face than its fellow. 

Periscopic Glasses. —In the effort to further apply the 
law requiring that the plane of the lenses shall be perpen¬ 
dicular to the visual axes, we are met with the fact that 
with biconvex and biconcave lenses this relation is only 
strictly possible within a comparatively limited area sur¬ 
rounding the optical center of the lens. When the wearer 
looks through the periphery of his glasses the visual axes 
will pierce the lenses obliquely, and the refractive value of 
the latter will, of course, be governed by all the laws of 
tilted lenses. For instance, when the wearer of an ordinary 
convex lens looks through it near the edge, the optical 
effect of the glass before his eye is that of a stronger con¬ 
vex lens combined with a cylindrical lens; the axis of the 
latter depending on the part of the periphery pierced by 
the line of sight. In weak lenses, the slight inaccuracy of 
vision produced in this way is of small moment, but 
where the strength of the lens used is greater than about 
2. D. the patient’s field of accurate vision is greatly reduced 
in size, and in viewing objects not directly in front of him 
he is obliged to perform wide motions of the head in order 
to be able to see them through the central portion of his 


5o 


SPECTACLES AND EYEGLASSES 


glasses. This is especially true of cases of aphakia, where, 
of course, very strong lenses are generally necessary. To 
escape or lessen these disadvantages, strong spherical 
lenses should be, and generally are, made in the form of a 
meniscus, which when placed with its convex surface from 
the eye constitutes a periscopic glass. The ideal of this 
form of lens may be defined as a glass in which the center 
of curvature of one surface coincides with the center of 
rotation of the eye, and that of the other surface 
approaches it as closely as the required strength of the glass 
will permit. In such a glass the visual axis will always be 
perpendicular to the first surface, and nearly so to the 
second, at whatever point it pierces the glass, and in what¬ 
ever direction the eye may be turned. 

When a cylindrical or sphero-cylindrical lens is required, 
the best form of glass is the toric lens described on page 20. 
By transposing the usual formula, however, there may be 
obtained a sphero-cylindrical lens which approaches the 
periscopic form, and is certainly superior to one ground after 
the usual method. For illustration, if one desires to order 
+ 2. D. Sph. O + .75 D. Cyl. Ax. 90°, the formula may 
be transposed and the order written for + 2.75 D. Sph. O 
— .75 D. Cyl. Ax. 180 0 . This glass, though optically of 
the same strength as the first, would have an approach 
to the periscopic form if placed with the cylindrical 
surface next the eye: The field of accurate vision would 
gain in all directions, especially in the vertical one, in 
which diameter, however, its enlargement is not of so much 
consequence as it is laterally. Aphakic eyes offer the best 
field of usefulness for this practice, as in them we have 
generally to deal with a high hyperopia, and often with 
hyperopic astigmatism requiring for its correction a con¬ 
vex cylinder with its axis horizontal. Let us suppose that 
after a cataract extraction we wished to order + 10 D. 
Sph. O + 6. D. Cyl. Ax. 180°. With this lens accurate 
vision would be limited to a vertical oval field situated 


THE PRINCIPLES OF SPECTACLE FITTING 51 

directly in front of the patient, beyond the confines of 
which all objects would appear distorted by various cylin¬ 
drical effects. We would, therefore, transpose the formula 
into + 16. D. Sph. O — 6. D. Cyl. Ax. 90°, and this glass 
will be likely to give the patient much more satisfaction 
than the other would have done, as with it he obtains a 
very good lateral field. 


III. PRESCRIPTION OF FRAMES 


In order to prescribe the frames for a pair of spectacles, 
we must, after measuring the face or a frame which fits, 
record the dimensions of the frame we desire to order. 
The essential measurements are the intercentral distance, 
or width of front, and the three dimensions of the bridge. 
This list may be extended to include the measurement of 
the angle formed by the crest of the bridge and the plane 
of the lenses, that formed by the temples and the plane of 
the lenses, the distance between the temples an inch back 
of the glasses, and the distance from the hinge of the tem¬ 
ples to the top of the wearer's ear. All these details are, 
however, so ready of adjustment, and the trouble and 
uncertainty of their prescription are so great, that in my 
judgment they are better left until the frame is received 
from the maker and we are ready to adapt it to the patient’s 
face. The distance between the centers of the spectacle 
eyes is best obtained by measuring upon the face the 
distance between the centers of the pupils; the other 
dimensions of the frame, however, are more easily obtained 
by trying on a sample frame and taking the measurements 
from this, estimating any change which may be necessary. 
To do this requires a half dozen sample frames of different 


Size of Eye 

Between 

Center 

Height 
of Bridge 

Top of 
Bridge In 
or Out 

Width 
of Base 

Length of 
Temple 

No. oo. 

66 mm. 

8 mm. 

1 mm. out 

20 mm. 

long (6 Y 2 in.) 

o. 

64 mm. 

6 mm. 

2 mm. out 

24 mm. 

medium (6 in.) 

i. 

62 mm. 

4 mm. 

2 mm. out 

21 mm. 

medium (6 in.) 

i. 

60 mm. 

2 mm. 

2 mm. in 

19 mm. 

medium (6 in.) 


57 mm. 

3 mm. 

1 mm. in 

16 mm. 

medium (6 in.) 

3 . 

55 mm. 

1 mm. 

0 mm. 

15 mm. 

short in.) 




















PRESCRIPTION OF FRAMES 


53 


dimensions in their different parts. With such an assort¬ 
ment as is here given, which is a very good one, a part 




nearly or exactly of the required size can always be found 
in one or the other of the frames. It may, of course, be 
necessary to get the height of bridge from one frame, its 





































54 


SPECTACLES AND EYEGLASSES 


breadth from another, and the length of temple from still 
a third. 

A rule graduated in millimeters or sixteenths of an inch 
is also necessary. 

I have had made for this purpose a rule which I think 
facilitates the work. As represented in Fig. 31, it has 
upon one side three scales graduated in millimeters and 
conveniently placed for taking the different dimensions of 
the frame, while on the reverse side are several ovals show¬ 
ing the principal sizes of spectacle eyes. Some of the uses 
of these scales are shown in Figs. 32, 33, 37 and 38; to 
avoid confusion one scale only is drawn in each diagram. 

! 

Philadelphia, .19 


Name of Patient, . 

O.D . 

0 . S .:. 

Frames of 

Inter pupillary Distance. 

| Height . 

Bridge j 

Width of Base. 


. M. D. 

A prescription blank such as that here given indicates 
what measurements are required, and will be found useful 
in practice. The upper part is for the lenses, the lower 
part for the frames. 


Catalogue No. .. 

r ^ in 

. lop 

out 
























PRESCRIPTION OE ERAMES 


55 


To Obtain the Interpupillary Distance, with which 
the first dimension of the frame, the distance between the 
geometrical centers (A to B, Fig. 32) is generally identical, 
the physician seats himself facing the patient in a good 
light, the latter being directed to look straight before him 
at some distant object. The measuring rule is placed 
before the patient’s eyes, as close to them and as far from 
the physician’s eyes as possible. The zero of the scale 
being placed opposite the center of one pupil, the center of 
the other may be marked by the physician’s thumb nail, as 
represented in Fig. 33, and the distance between them read 
off the scale. This distance seldom varies more than 5 mm. 



from 60 mm., or 2% in. It will be observed that as the 
physician’s eyes are less than the length of his arm away 
from the patient’s face when this measurement is taken, 
in fact, about two feet away, the marks upon the rule, 
though apparently opposite the pupils, will in reality be a 
little within the centers; so that the distance obtained will 
be a little less than it should be. When the physician’s 
eyes are two feet away from those of the patient, and the 
rule is one inch away from them, the error in measuring an 
interpupillary distance of 60 mm. by this method is almost 
exactly 2 mm. This amount should, therefore, be added 
to the apparent interpupillary distance to obtain the true 
one. 

The measurement obtained in this way is sufficiently 
accurate for most purposes, but if a greater degree of accu¬ 
racy be desired in any case it may be attained by means 
of the little device suggested by Dr. Maddox, which is 










SPECTACLES AND EYEGLASSES 


56 


represented in Fig. 34. This is to be placed before one of 
the patient’s eyes in an ordinary trial frame having a gradu¬ 
ated bar for showing the distance of each geometrical 



center from the middle of the bridge. The gaze of the 
observed and that of the observing eye being directed to 
each other’s pupils, the two sights of the implement are 
brought into line between them as shown in Fig. 35. The 



Tig. 35 -—The pupil localizer in use. 


same procedure is then gone through with for the other 
eye, and the distance of the second pupil from the median 
line of the face, as registered by the trial frame is added to 
that of the first, to obtain the interpupillary distance. 




















































































PRESCRIPTION OF FRAMES 


57 


This procedure is also of advantage in revealing and meas¬ 
uring any difference in the distance of the pupils from the 
median line, due to asymmetry of the face. The use of a 
trial frame for making accurate measurements requires the 
bestowal of considerable attention to see that the support 
of the nose-piece is vertical, the joints close and tight, and 
the markings correct; otherwise it may readily introduce 
the errors its use is intended to obviate. There are, in 
the shops, many special forms of the “ pupillometer con¬ 
structed on the principle of a rule held before the eye^ 
and a single sight for each pupil. One of these is shown m 
Fig. 36. The interpupillary distance as registered by 
it requires, of course, the same correction as does that 
obtained by the simple graduated rule. 



T y 

Fig. 36. 

Height of the Bridge. —This is the distance of the top 
of the bridge above a line joining the centers of the lenses. 
In Fig. 32, it is the distance from E to F, which is the 
height of E above a line joining A and b; not the height 
of E above a line joining c and D, which is sometimes 
erroneously supposed to represent the height of the bridge. 

If a rule be held horizontally before the patient s eyes, 
with the lower edge touching the nose at the natural posi¬ 
tion for the spectacle bridge, the height of this edge ot the 
rule above the pupil on either side will show at a glance 
about how high the top of the future bridge must be. We 
may then select from our sample frames that one whose 
bridge corresponds most nearly with this supposed height, 
and being sure to place it in the natural position, we care¬ 
fully note whether the pupils are above or below the 
centers of the eyes of the frame. If they are below these 
centers, sufficient must be added to the height of the bridge 










































58 SPECTACLES AND EYEGLASSES 

now upon the face to allow them to coincide; if the pupils 
are above the centers, a corresponding subtraction from 
the height of the trial bridge must be made. Each sample 
frame may have its dimensions attached to it, or any frame 
may be used as a fitting frame and afterward measured. 
To measure the height of a bridge the glasses are laid 
upon a sheet of ruled paper, or other object offering a con¬ 
venient straight line, in such a way that the line passes 
through the geometrical centers of the eyes, or, what is the 
same thing, through the joints of the end pieces on each 



side (Fig. 32). The height to which the bridge projects 
above this line is then readily measured. It is seldom 
greater than 10 mm., and in rare cases may be a minus 
quantity, the top of the bridge being below the level of 
the centers of the lenses. 

Relation of the Top of the Bridge to the Plane of the 
Lenses. —The measurement required to express this 
relation is that from J to K in Figs..37 and 38; not the 
distance of J in front of a line joining C and D, as might 
be supposed. This measurement is also shown at H /, 
Figs. 26 and 27; it is obtained by a procedure similar to 
that just described for obtaining the height of the bridge. 
The rule being placed across the nose at the natural point, 
and the patient requested to wink, it may readily be seen 
whether the lashes touch the edge of the rule. If they do, 














PRESCRIPTION OF FRAMES 


59 


the top of the bridge of the future spectacles must be back 
of the plane of the glasses, or “in.” If they do not, we 
note how much nearer, if any, the edge of the rule might 
be brought without their touching, and so obtain a guide to 
the distance the top of the bridge should be in front of the 
plane of the lenses, or “out.” The fitting frame which 
comes nearest to the requirements of the case in this partic¬ 
ular is then placed upon the face, when by viewing it from 
above or from the side it can quickly be seen just how 
much change, if any, is needed to place the glasses a little 
beyond the reach of the lashes. The method of measuring 
the distance of a bridge in or out is so plainly shown in 



Figs. 37 and 38 that special explanation is unnecessary. 
They seldom measure more than 4 mm. out or 3 mm. in. 

Width of Base— The measurement from C to 79 , 
Fig. 37, is obtained, like the others, by measuring a bridge 
which fits, or estimating the change necessary in one which 
does not. This dimension is usually from 16 mm. to 20 
mm. 

Angle of the Crest of the Bridge. —It is not usually 
necessary to prescribe this angle, for the reason that the 
line formed by the bones of the nose, as seen in the profile 
of the face, is nearly always vertical at its upper portion 
and its direction changes so as to approach the horizontal 
as the nasal bones expand and jut forward to form the 
bridge of the nose. The direction of some portion of this 

















6o 


SPECTACLES AND EYEGLASSES 


line will coincide with the flat under surface of the top of 
the spectacle bridge, and it is on that portion of the line 
that the spectacles will tend to rest. There are cases, 
however, of noses with a very straight and vertical outline, 
in which the flat wire forming the top of the spectacle 
bridge finds no suitable support and rests, more or less, on 
its posterior edge. In other cases, exaggerations of the 
aquiline type, the line of the crest of the nose turns sharply 
forward to a nearly horizontal direction and the wire of 
the bridge tends to rest on its anterior edge. In these cases 
it is well to prescribe the angle which the top of the bridge 
makes with the plane of the lenses. Mr. Merry, of Kansas 
City, has invented a little implement for quickly determin¬ 
ing this angle. Its appearance and the manner of its use 
are so well shown in Figs. 39 and 40 as to require no 
description. 

This method of obtaining the dimensions of the bridge 
required may seem tedious and uncertain in the descrip¬ 
tion; in use it is not so, and after trial I think will be found 
preferable to any special device so far invented for record¬ 
ing the measurements. These, after shifting of screws and 
bending of wires, leave one to estimate what changes are 
required just as might have been done without their aid. 
Moreover, the heavy parts and lost space in joints of trial 
frames may readily conceal an error of 2 mm., or even 3 
mm. in some measurements; the large, round eyes with 
heavy rims will not go under the brows, so that the in-out 
measurement of the bridge must frequently be guessed at; 
and the relation of the upper part of the eye wires to the 
brows is not shown. In fact, they introduce, in my 
estimation, quite as many sources of error as they eliminate. 

Where the face is unsymmetrical no exact rules of pro¬ 
cedure can be given, and considerable ingenuity may be 
required to fit a frame to such a face. If the nose is very 
peculiar, or one side of its bridge markedly steeper than 
the other, it may be of advantage to take an outline of the 


PRESCRIPTION OP FRAMES 


61 


bridge at the natural position by bending a piece of lead 
wire to fit accurately and marking the outline of this upon 
the prescription blank, or sending the wire itself to the 
spectacle maker. Sometimes the brows are overhanging 
and the eyes deep set; so that the glasses cannot be prop¬ 
erly centered before the eyes and placed close to them 
without the upper part of the rims burying themselves in 
the brows. In such cases the glasses should be decentered 



Fig. 39- 



Fig. 40. 


upward in their frames and the bridge made sufficiently 
high to bring the optical centers opposite the pupils. 
Though the patient will then look through the upper part 
of his glasses, his field of vision will not be any more limited 
than is already the case because of the overhanging brows. 

Prescription of Eyeglasses.— The dimensions which it 
is usual to furnish in prescribing eyeglass frames are the 
interpupillary distance, of course, with the distance 







62 


SPECTACLES AND EYEGLASSES 


between the two upper and the two lower ends of the nose- 
pieces when they are in place on the face (Fig. 61). 
These measurements alone will not insure a good fit 
in the frames, since neither the contour of the sides of 
the nose to which the guards are applied, the vertical cen¬ 
tering of the lenses, nor the distance of the latter from the 
eyes are taken into account; but the same remark applies 
here as to the minor dimensions of spectacle frames, namely 
that it is more simple, certain, and expeditious for the 
surgeon to make these adjustments in the frames them¬ 
selves than to prescribe what the manufacturer shall do for 
him. A series of three or four frames with variations in 
the length and shape of the spring and in the pattern of the 
guards is sufficient for trying on. Fortunately, eyeglass 
frames admit of great variation by bending their different 
parts, and being put together with screws, these parts are 
quickly interchangeable. Almost the only thing about 
them which admits of no adjustment is the length of the 
spring, and it is well for one who prescribes many eyeglass 
frames to have a series of such springs at hand from which 
to replace one which may be found too long or to'o short. 


IV. INSPECTION AND ADJUSTMENT OF 
SPECTACLES AND EYEGLASSES 

Ordinary prudence demands that the prescriber of 
glasses make a careful examination of the manner in which 
his directions have been carried out, since neglect of this 
precaution may nullify the results of the most painstaking 
correction of the refraction. If the surgeon himself furnish 
the spectacles, it is doubly incumbent on him to make a 
thorough inspection of glass and frame, and to carefully 
adjust the latter so as to be entirely comfortable to the 
wearer. Then, too, it is not enough that the frames cor¬ 
rectly perform their function at first; they must continue 
to do so. Should there be no optician in his neighbor¬ 
hood, the surgeon will be called upon to bring to a proper 
shape frames which have passed through all sorts of acci¬ 
dents, and it is better that he should do this work than 
entrust it to less competent hands. 

Proving the Strength of Lenses. —The focal length of 
a convex lens may be directly measured by finding the 
distance at which it brings the sun’s rays to a focus. I o 
do this, the rays which have passed through the lens are 
simply caught upon a piece of paper or other screen, the 
two being held in such relationship that the image of the 
sun formed on the screen is round. The screen is then to 
be moved back and forth until the point is found at which 
this image is smallest, and the distance of such point from 
the lens is the focal length of the lens. To learn the 
strength of the lens in diopters, we divide ioo centimeters 
(one meter) by the focal length expressed in centimeters, or 
40 inches (about one meter) by the focal length expiessed 
in inches. For instance, if we found the focus of the lens 

63 


64 


SPECTACLES AND EYEGLASSES 


under examination to be distant io in., or 25 cm., from the 
lens, 40 in. divided by 10 in., or 100 cm. divided by 25 
cm., will alike give a quotient of 4, and the lens measured 
was, therefore, a + 4. D. 

The focal length of a concave lens may be similarly mea¬ 
sured by combining it with a stronger convex lens and then 
measuring the strength of the resulting weaker convex. 
The strength of the original convex used being known, we 
have only to subtract from it the weak convex resultant 
to find the strength of the concave with which we are deal¬ 
ing. The focal length of convex and concave cylindrical 
lenses may be measured in the same way as the correspond¬ 
ing sphericals, it being only necessary to observe that the 
parallel rays of light after passing through a convex cylin¬ 
drical lens are arranged in the form of a line at the focus of 
such lens; not brought to a point, as is the case with con¬ 
vex sphericals. 

Phacometers. —Such methods as the one described 
above are, however, too tedious for ordinary use, though 
quite elaborate contrivances called phacometers have been 
devised on this principle. A lens measure constructed on 
an entirely different idea has appeared, the invention 
of Mr. J. T. Brayton, of Chicago. Fig. 41 shows the size 

t 

and appearance of the instrument, as well as the method of 
its use. Of the three steel pins which project from its top 
the two outer ones are fixed, while the central one moves 
up and down easily but is held up by a spring. On press¬ 
ing the surface of a spherical lens squarely against these 
points the central one will be depressed until they all 
three touch the glass, the curvature of the surface of the 
lens determining the amount of such depression. The 
motion being *ansferred through a rather simple mechan¬ 
ism to the hand upon the dial, this travels over a scale 
which shows in diopters the strength of the lens corre¬ 
sponding to the surface tested. The other surface is then 
to be explored in the same way. If the lens is bi-convex 


INSPECTION AND ADJUSTMENT OF SPECTACLES 65 

or bi-concave, the results of measuring each surface 
separately are added together; if periscopic, the less is 
deducted from the greater. When used upon a cylindrical 
surface the hand will stand at zero when the three points 
are in line with the axis of the cylinder. When the points 
are placed at right angles to the axis the strength of the 
cylinder is shown. 

Since this instrument indicates the refractive value of a 


Fig. 41. 

lens from the curvature of its surfaces only, leaving out of 
account the index of refraction of the material, it is evident 
that it can be accurate for only one variety of glass. To 
adjust the instrument turn the midmost of the three 
points to either the right or left until the index properly 
registers the value of a known lens. It will then be adjusted 
for all lenses made of a like glass. 

Neutralization of Spherical Lenses. —The method of 
determining the strength of spectacles which is of most gen¬ 
eral utility is the well-known one of neutralization. If a 
convex spherical lens be held about a foot from the eye, 

and any object, say that part of a window sash where a 
5 





















66 


SPECTACLES AND EYEGLASSES 


vertical and horizontal line cross, be viewed through it, any 
motion given the lens will result in an apparent motion in 
the opposite direction of the object sighted. That is, if the 
lens is moved to the right, the object appears to move to 
the left; if the lens is raised the object appears to sink. If 
the same maneuver be employed with a concave spherical 
glass, the object again appears to move, but this time in the 
same direction as the motion imparted to the lens; if the 
lens is moved to the right, the object appears to move to 
the right also. Here we have the readiest possible means 
of distinguishing between a convex and a concave lens. 
Moreover, one gets in this way an idea of the strength of a 
lens, as the stronger the lens the more rapid is the apparent 
motion of the object seen through it. 

If, continuing the experiment, the two lenses be placed 
together, with their curved surfaces in apposition, and a 
trial be made of the effect of moving them before an object, 
as was done previously with each lens singly, the object 
will appear: i (if the concave lens is the stronger), to move 
in the same direction as the motion of the glass, but more 
slowly than before; 2 (if the convex lens is the stronger), 
to move in the opposite direction to the motion of the 
glass, but more slowly than before; 3 (if the lenses are of 
equal strength), to have no motion. Therefore, to find the 
strength of a spherical lens it is only necessary to combine 
it in this way with successive lenses of known strength and 
of the opposite sign until that one is found which neutral¬ 
izes the apparent motion of objects seen through it. This 
lens is the measure of the strength of the one tested. This 
method is accurate within an eighth diopter, or less, for 
plano-convex and plano-concave lenses; with bi-convex, 
bi-concave, and toric glasses it is only possible to neutralize 
the apparent motion near the center of the lens; toward the 
edges motion will still be visible when the lenses are strong. 

Cylindrical Lenses may be recognized by viewing 
through them some object presenting a straight line, say 


INSPECTION AND ADJUSTMENT OE SPECTACLES 67 

the vertical line of a window sash. If the cylindrical lens 
be rotated about the visual axis, the portion of the vertical 
line seen through the glass will appear to be oblique, as 
compared with that seen above and below the glass (Fig. 
42). This oblique displacement takes place in a direction 
contrary to the rotary motion given the lens if the latter is 
convex, and in the same direction as the motion if the lens 
is concave. To ascertain the position of the axis of a 
cylindrical lens it should be rotated slowly in this manner 
until the line seen through it appears continuous with that 
above and below (Fig. 43). This line will then lie either 




Fig. 43. 


in the axis or at right angles to it. To ascertain which of 
the latter is the case, the effect of motion from side to side 
is to be tried. If the axis of the cylinder corresponds with 
the vertical line looked at, motion from side to side pro¬ 
duces apparent motion of the object; if, however, the axis 
lies at right angles to the vertical line no such motion results. 
In other words, in the direction of its axis a cylindrical lens 
acts as a piece of plain glass; across its axis it acts as a 
spherical lens of the same strength. If it is desired to 
know upon which surface of a lens the cylinder is ground, 
this may be ascertained by holding the lens nearly hori¬ 
zontally between the eye and a window, so that the line 















68 


SPECTACLES AND EYEGLASSES 


of sight strikes its upper surface very obliquely. One can 
thereby see the lines of the window reflected upon the 
upper surface of the lens. By rotating the lens about its 
optic axis these lines appear broken if the surface is cylin¬ 
drical, but retain their continuity if the reflecting surface 
is spherical. The direction of the axis of a cylindrical lens 
• having been ascertained, its strength may be determined 
by neutralizing it with a cylinder of the opposite sign, as 
was explained when speaking of spherical lenses. Care 
must be taken that the two lenses are so placed that their 
axes coincide. 

A Sphero-cylindrical Lens is equal in refractive effect 
to two cylindrical lenses with their axes perpendicular to 
each other. Having found that axis across which motion 
is least rapid, we may neutralize the motion with a spher¬ 
ical lens and, holding these two together, proceed to neu¬ 
tralize the motion across the other axis just as if dealing 
with a simple cylinder. When our object is not to deter¬ 
mine the strength of an unknown lens, but to see if the 
lenses of a pair of spectacles agree with the prescription 
previously written, we may, of course, shorten the above 
procedures by picking out from the test case the glass, or 
glasses, which will neutralize the spectacles if the latter are 
of the proper strength, and observing whether the apparent 
motion of objects ceases when they are held together. 

Locating the Optical Center. —Every glass before being 
worn should be examined with regard to the position 
of the optical center of each lens and the distance of these 
from each other, as inaccuracy in this important particular 
is not uncommon. Indeed, in the cheap spectacles which 
some persons unfortunately buy, proper centering is the 
exception. In grinding large numbers of lenses by machin¬ 
ery a certain number in each batch are, I believe, always 
found to be badly centered. These are not returned to the 
wheel or the furnace by the thrifty manufacturer, but are 
graded as second class, or if very bad indeed as third class, 


INSPECTION AND ADJUSTMENT OF SPECTACLES 69 

and with those which will not pass inspection in other 
particulars go to make up the trash sold by peddlers. 

A simple way to find the location of the optical center is 
to hold the lens about a foot above the corner of a rectan¬ 
gular card lying on the table. The corner seen through 
the lens will only appear complete and continuous with the 
rest of the card when its tip is opposite the optical center. 

In Fig. 44, a represents a lens so held that its optical 
center is marked by the corner of the underlying card; b 
is a lens improperly held. The center first found may be 
marked with a speck of ink, the center of the other spec¬ 



tacle glass found in the same way, and the distance 
between them measured. If care is taken to hold the glass 
exactly level and the eye directly over it this method will 
give results accurate enough for most purposes. 

The Apex of a Prism may be determined by viewing 
through the glass fine lines crossed at right angles, holding 
the prism so that its edge and supposed apex just touches 
one line at the point of intersection. When the real apex 
of the prism coincides with the intersection of the lines, the 
appearance presented is that shown in Fig. 45; when, how¬ 
ever, the apex is to one side of the point of intersection, the 
line seen through the prism appears broken, as in Fig. 46. 
In this case the prism is to be rotated until the line appears 








70 


SPECTACLES AND EYEGLASSES 


continuous, when the point of intersection of the lines will 
mark the apex of the prism. 

The Strength of a Prism may be expressed in two ways; 
either in degrees of the refracting angle, which is the angle 
forming the edge and separating the two refracting sur¬ 
faces of the prism, or by means of some formula which 
denotes the power of the prism to turn a ray of light from 
its course. This power is usually expressed in degrees 
of the angle of deviation, which is the angle separating the 
course of a ray of light after having passed through the 
prism from that which it would have pursued had its course 
been unobstructed. The obvious advantage of the latter 




Fig. 45. Fig. 46. 

Figs. 45 and 46.—Method of finding the apex of a prism. 


{After Maddox .'i 


mode of expression, which gives directly the optical strength 
of the prism, over the former, which merely states the 
value of a physical angle from which the strength can 
be more or less accurately inferred, has called forth several 
suggestions for an improved method of numbering ophthal- 
mological prisms. Dr. Edward Jackson was the first to 
point out that the prism had escaped attention when the 
numeration of our other glasses was reformed. He pro¬ 
posed that in harmony with the mode of stating the value 
of angles which is commonly accepted in other depart¬ 
ments of science, they be marked in degrees of their angles 
of deviation. With the idea of conforming their numera¬ 
tion to the dioptric system of numbering lenses, Mr. C. F. 
Prentice proposed to adopt as a unit that prism having 
the power necessary to produce one centimeter of devia¬ 
tion in the course of the ray after having passed through 
and the distance of one meter beyond the prism. Dr. 






INSPECTION AND ADJUSTMENT OF SPECTACLES 71 

S. M. Burnett proposes that this unit be called the prism 
diopter, and that the centimeter of deviation be measured 
upon a plane surface—that is, upon a tangent of the arc 
whose radius is one meter. 

Within practical limits the objections which have been 
raised to the prism diopter are few and of little moment, 
and it has great simplicity to recommend it. In a series 
of prisms so numbered, however, the higher prisms are not 
simple multiples of the lower ones. Twenty prisms of two 
P. D. each would not be equal to a prism of 40 P. D., but 
to a prism of 42 P. D. 

The centrad as a unit of measurement of prism power 
was suggested by Dr. W. S. Dennett. After mature con¬ 
sideration this unit has been formally recommended by the 
American Ophthalmological Society, and will doubtless in 
a few years entirely, as it has already to a great degree, 
displace the old system of numbering. 

The term radian denotes in mathematics a portion of the 
arc equal to the radius. The centradian is the one hun¬ 
dredth part of the radian. The centrad is such a prism as, 
held with one surface perpendicular to the incident ray, 
causes a deflection equal to a centradian. If the measure¬ 
ment be made at one meter, then, the radius and radian 
being each one meter long, the centradian will equal a 
centimeter, measured on the arc, and the centrad is such a 
prism as will produce this amount of deflection. If the 
measurement be made at two meters—a very convenient 
distance—one centrad will produce a deflection of one 
hundredth of two meters, or two centimeters. 

It will be seen that the practical difference between a 
centrad and a prism diopter consists in this, that in the 
former the amount of deflection is measured on the arc, 
while in the latter it is measured on the tangent. For 
ophthalmological prisms, which are of necessity weak, the 
difference between centrads and prism diopters is so slight 
as to be of no moment. The numeration of prisms by 




72 


SPECTACLES AND EYEGLASSES 


centrads has the advantage that it is founded on a method 
of stating the value of the angle which is used in other 
departments of physics. Its higher numbers in the scale 
are, moreover, simple multiples of the unit. 

Over the system of numbering prisms in degrees of the 
refracting angle the use of the centrad has all the advan¬ 
tages possessed by the modern numeration of spherical 
lenses over the old. Its use, moreover, involves no per¬ 
plexity in the mind of one who has become habituated to 
the former method, since, as shown in Table VI the differ¬ 
ence in value of one of the old and one of the new prisms 
of the same number is slight for the weaker, more used 
prisms. The one, however, represents a definite, fixed 
value; the other does not. 

As the surgeon has a choice of two essentially different 
methods of numbering, so, also, he has at his command 
several modes of determining the strength of unknown 
prisms, and may select that one which is simplest and 
involves least calculation for the numeration which he 
uses. The refracting angle may be readily found by 
means of Table III, introduced when speaking of the pris¬ 
matic equivalent of decentered lenses. The situation of 
the optical center is to be marked upon a spherical lens of 
convenient strength, and the prism to be tested super¬ 
imposed. By viewing the corner of a card through these 
two glasses, as was directed in describing the method of 
finding the optical center, this center will be found to have 
been carried toward the base of the prism. The position 
of this apparent optical center is to be likewise marked 
upon the spherical lens, and its distance from the true one 
measured. In the left-hand column of Table III find the 
strength of the lens used, and on a level with this across 
the page the distance in millimeters between the true and 
apparent optical centers. At the head of the column in 
which this measurement is found will stand the strength of 
the prism with which the lens was combined, this strength 


INSPECTION AND ADJUSTMENT OF SPECTACLES 73 

being expressed in degrees of the refracting angle. For 
instance, if having combined an unknown prism with a -j- 

Table VI. —Showing the Equivalence of Centrads in Prism Diopters 
and in Degrees of the Refracting Angle (Index of Refraction 

i-54) 


Centrads 

Prism Diopters 

Refracting Angle 

1 

1 

i°.oo 

2 

2.0001 

2°. 12 

3 

3.0013 

3 °. 18 

4 

4.0028 

4 °-23 

5 

5-0045 

5 0 .28 

6 

6.0063 

6°.32 

7 

70115 

7°-35 

8 

8.0172 

8°.38 

9 

9.0244 

9°-39 

10 

10.03 

io °.39 

11 

11.044 

n °-37 

12 

12.057 

12 0 .34 

13 

13-074 

I 3 °-29 

14 

14.092 

I 4 °-23 

i 5 

i 5 -ii 4 

I5°.i6 

16 

16.138 

i6°.o8 

17 

17.164 

i6°.98 

18 

18.196 

17 °.85 

19 

19.230 

i8°.68 

I 9 °- 45 j 

20 

20.270 

25 

25-55 

23°-43 

30 

30-934 

26°.81 

35 

36.50 

29 0 .72 

40 

42.28 

32°.18 

45 

48.30 

34 0 .20 

50 

54-514 

35°-94 

60 

68.43 

38°.31 

70 

84.22 

39°-73 

80 

102.96 

40°.29 

90 

126.01 

40°.49 

100 

155-75 

39 °-i 4 


7. D. lens we find the apparent displacement of the optical 
center to be 4 mm., the table shows at a glance that the 
refracting angle of the prism tested had a value of 3 0 . 














74 


SPECTACLES AND EYEGLASSES 


The refracting angle may be directly measured by 
adapting the legs of a pair of compasses to the two refract¬ 
ing surfaces and then laying the compasses on an ordinary 
protractor. 

The surgeon is, however, very little concerned with the 
refracting angles of prisms, except as they are the basis of 
the old system of numbering, which is now almost super¬ 
seded by one in which the number of the prism indicates in 
centrads the power which that prism possesses of causing 
deviation in a ray of light. One of the simplest and most 
convenient devices for measuring this power is that 
suggested by Dr. Maddox. It consists of a strip of card- 



Fig. 47. 


board suspended horizontally on the wall on a level 
with the eyes of the observer. The upper border of the 
card (Fig. 47) is marked from right to left with a scale of 
degrees, or rather tangents of degrees, proper to the dis¬ 
tance at which the prism is to be held from the card. In 
Table VII is given the distance from the right-hand border 
of the card of the mark for each degree of deviating angle. 
With the help of this table one may readily construct the 
scale, using column A if he elect to work at six feet, or 
column B if a two-meter range be preferred. 

To practice this method of prismetry, the glass to be 
tested is held at the proper distance from the card, its apex 
to the left, and its upper border just below the figures of 
the scale, as in Fig. 47. The observer’s eye being placed 
behind the prism, the right vertical border of the card 
appears displaced toward the observer’s left and points 
upward to the number expressing the strength of the 
prism in degrees of the angle of deviation. During this 








INSPECTION AND ADJUSTMENT OE SPECTACLES 75 


maneuver care must be taken that the prism is held at pre¬ 
cisely the distance from the card for which the scale of the 
latter is arranged; also that the apex of the prism points 
exactly to the left. This latter requirement may be 

Table VII* 


For Marking a Card in Tangents of Degrees at 6 Feet (Column A); or 

2 Meters (Column B) 



A 

B 


A 


B 

1° 

1.25 in. 

3.49 cm. 

9 ° 

11.4 in. 

31.29 cm. 

2° 

2.5 in. 

6.98 cm. 

IO° 

12.6 

in. 

34 73 cm. 

3 ° 

3.7 in. 

10.467 cm.l 

ii° 

14.0 

in. 

38.16 cm. 

4 ° 

5.0 in. 

13.95 cm. 

12° 

15-3 

in. 

41.58 cm. 

5 ° 

6.3 in. 

17.43 cm. 

13 ° 

16.6 

in. 

44.99 cm. 

6° 

7-57 in. 

20.9 cm. 

14 ° 

17.9 

in. 

48.38 cm. 

7 ° 

8.84 in. 

24.37 cm. 

15 ° 

19-3 

in. 

51.76 cm. 

8° 

10.12 in. 

27.83 cm. 

16° 

20.64 in. 

55-13 cm - 


secured by rotating the prism until the line of the bottom 
of the card appears unbroken, as at A, in Fig. 47. In 



Fig. 49. 


adapting this method of prismetry to centrads or prism 
diopters, the scale at the top of the card should simply be 
laid off in centimeters, and the prism be held at the dis- 


* From Maddox: “The Clinical Use of Prisms.” 























76 


SPECTACLES AN 1 ) EYEGLASSES 


tance of one meter. Each centimeter that the right border 
of the card is apparently moved to the left, on viewing it 
through the prism, will then represent one centrad, or 
one prism diopter. 

Scratches, Specks, Bubbles, Flaws, etc., in the glass 
will hardly escape detection if they are carefully looked for 
while the lens is held in different lights. Placing the glass 



against a dark background and allowing a bright light to 
fall obliquely upon it will perhaps bring them out as plainly 
as any other maneuver. 

Irregularity of the Surface may be discovered by reflect¬ 
ing from that surface any object having regular outlines. 
The observer should stand facing a window, holding the 
lens against a dark background in his left hand, and pass a 



straight-edged piece of paper held in his right hand between 
his eyes and the lens. Two images of the paper will be 
reflected from the lens—one formed by each surface. 
Any irregularity of these surfaces will make the images 
appear broken, or with wavy outlines. 

Adjusting Spectacle Frames. —It requires some little 
practice to enable one to tell at a glance just where such 
an irregularly shaped object as a spectacle frame has been 















INSPECTION AND ADJUSTMENT OF SPECTACLES 


wrongly bent; having found the error, it is a more simple 
matter to correct it. For the latter purpose small pliers 
are required. They should have narrow but strong 
jaws, round in one pair and square in another. (Fig. 48.) 
For different parts of the frames and for making special 
bends many special forms of pliers are in use. Fig. 49 
shows the shapes of the jaws of a few of them. A small, 
stout screw-driver with a point suited to the screws of 
spectacles will also be necessary. A special pencil which 
makes a white, easily erasable mark on glass is useful for 
marking the position of centers, and of axes of cylinders. 
Such marks are a great aid in fitting frames. 



Eye wires are generally of such light material as to take 
their shape from the contained glass, and are, therefore, not 
liable to become misshapen. The popular round lenses 
frequently rotate within the eyewire, which is disastrous 
in case they contain a cylinder or prism. On such a lens 
near its margin two minute diamond marks should be 
placed to show the horizontal diameter. Sometimes the 
long axis of an oval eye gets rotated within the eye wire 
(Fig. 50), so that it no longer stands squarely across 
the face. By loosening the screws it can readily be 
re-adjusted. Abnormal crookedness about the bridge is 
best disclosed by placing a straight edge (indicated by the 









7 S 


SPECTACLES AND EYEGLASSES 


line S E in Figs. 50, 51, 53, 54 and 55) in such a position 
as to enable one to compare the two sides of the frame. 
If the bridge is bent at its junction with the eye wire a 
rotation results, looking very much like that just men- 



Fig. 53. 




tioned, but dependent upon an entirely different fault 
(Fig. 51). It is readily corrected with pliers or fingers. 

The planes of the glasses may cross each other (Fig. 52), 
in consequence of a twist in almost any part of the bridge 
though the trouble is, usually, that the angle of the bridge 
at A is not of the same size as its fellow of the oppos ; 
























INSPECTION AND ADJUSTMENT OF SPECTACLES 79 

side. The bridge is inclined, as shown in the cut, more to 
one glass than to the other. It requires application to the 
patient’s face to determine which is the proper inclination, 
and in order that the glasses may be equalized at this and 
not at the improper one. 

In Fig. 53 the bend is at the junction of the eye wire 
with the bridge, rendering corresponding angles of the two 
sides of the frame unequal. The diagram shows the 
change necessary to correct the trouble. A similar fault 
is shown in Fig. 54. This appears at first sight to be just 
like the last; it is, however, a neighboring angle of the 
bridge which needs equalizing with its fellow. 

In the frame represented in Fig. 55 the glasses lie in 
the same plane, but one of them is nearer the center of the 
bridge than the other, due to the fact that, of the angles 
of the bridge which can be seen by viewing the frame in 
this position, the two which lie on one side of the curved 
portion are too much open, while the two on the other side 
are too little so. Of course, the bridge may be misshapen 
in any portion of its extent, but the illustrations given are 
sufficient to show the sort of faults one may expect. 

Having rectified all want of symmetry in the “front,” 
the defects in the fit of the temples can best be corrected 
by trying the frames on the patient’s face. If on doing so 
it is found that their temples cut into the temples of the 
wearer, instead of just touching the skin, as they should do, 
the trouble is obviously that the distance between the tem¬ 
ples is too small, and they must be bent out at the hinges, 
so as to throw them, when open, farther apart. This is 
done with the square-jawed pliers, seizing the wire close 
up to the hinge. When the opposite condition pertains, 
that is, when the distance between the temples is too great, 
leaving a space between each wire and the side of the 
wearer’s head, they require to be bent in. To do this, take 
the end of each side in turn in the square-jawed pliers, in 
such a way that the edge of one jaw shall be in contact with 


8 o SPECTACLES AND EYEGLASSES 

the temple as close to the hinge as possible and the latter 
be held rigidly open. The temple may then be pressed in 
with the fingers, and will bend at the point where it is 
pressed against the edge of the pliers. If the latter are 
rightly placed this does not make an angle in the wire form¬ 
ing the temple, but simply alters the angle already formed 
at A in Fig. 55, by the expansion of the end of the temple to 
help form the hinge. Care must be taken that one temple 
is not bent out more than the other, or, as is apt to be 
the case, become so during use. When this happens the 
effect is quite different from what might be expected. The 
glass on the same side as the temple the more bent out 



will be brought closer to the eye, while its fellow will be 
carried farther forward and the bridge will ride obliquely 
across the nose. To remedy this it is only necessary to 
equalize the divergence of the temples. 

The curve of hook temples given them by the maker 
will rarely be found to fit comfortably behind the ear. As 
was pointed out by Dr. Charles H. Thomas, the proper 
form for hook temples is a straight line from the hinge to 
the top of the ear, where a sharp curve should join this 
part of the temple to the easy curve which corresponds to 
the back of the ear (Fig. 56). Where the curve given the 
hook is too wide and is extended upon that part of the wire 
resting against the patient’s temple, as shown by the 
dotted line in Fig. 56, there is a constant tendency of the 
spectacles to slide forward. The wire, moreover, touches 


INSPECTION AND ADJUSTMENT OF SPECTACLES 81 

the back of the ear for a short distance only, where its 
pressure is further increased by the fact of the whole tem¬ 
ple being put upon the stretch and acting as a spring. 
Especially at first should the frames not fit too tightly, as 
the skin is then more easily irritated by the wire than when 
it becomes accustomed to its presence. 

In persons whose ears stand out far from the head a 
certain ridge upon the cartilage of the ear is thrown into 
prominence. Since the curve of a hook temple is a regular 
one, it will rest upon this ridge and be very uncomfortable; 
indeed, it may cut through the skin and into the cartilage. 
Under such circumstances the portion of the wire which is 
behind the ear should be made to follow every depression 
and elevation of the surface with which it is in contact; as 
it should in any case where the auricle is deformed or 
irregular in any way. 

If one lens stands higher upon the face than the other, 
so that the patient looks through the upper part of one 
glass and the lower part of the other, it will be found that 
the temple on the side which stands the higher is turned 
down more than its fellow. It should be raised, or more 
frequently its fellow should be lowered. The fault may 
lie in the bridge, as shown in Fig. 52, or in the end piece, 
or in the temple itself. In the first instance, bringing the 
lenses into the same plane removes the difficulty; in 
the second, take the end piece in the round-jawed pliers, the 
jaws being applied to its edges close up to the eye wire. 
Holding these pliers in the left hand, apply the square jaws 
of the other pliers to the surfaces of the end piece; when, 
by twisting the latter about its long axis, the temple may 
be turned down to any desired extent. Thus, the temple 
is not bent at all, but the end piece between the hinge and 
the eye wire. Nearly the same effect may be produced by 
bending the wire of the temple close up to the hinge. As 
was remarked before, in speaking of the facing of the glasses, 

the effect of turning down both temples is not to make both 
6 


82 


SPECTACLES AND EYEGLASSES 


lenses stand higher upon the face but to make the glasses 
face more downward. ^ 

Sometimes when the glasses do not sit properly the 
trouble will be found to be not in the frames but in the 
wearer. A considerable amount of asymmetry of the two 



Fig. 57 . 


sides of the face is not uncommon. One ear or one eye 
may be higher than its fellow, either of which conditions 
will make the glasses seem awry, and render necessary a 
compensating asymmetry of their frames. 

Adjustment of Eyeglasses. —The starting-point in 
adjusting eyeglasses is at the nose-pieces, whose free sur- 



Fig. 58. 


faces should be made to conform accurately to the bones of 
the nose by which they are supported. When received 
from the maker they are generally curved, presenting a 
convexity toward the nose. As the bones of the sides of 
the nose at the point where the guards are to rest are 
usually more or less convex also, the bearing obtained is 








INSPECTION AND ADJUSTMENT OF SPECTACLES 83 

a most insecure and uncomfortable one, as a glance at Fig. 
57 will show. In Fig. 58 this glass is shown with its nose- 
pieces properly adapted to the sides of the nose. Any con¬ 
formation may be required, but that shown in Fig. 58 is 
the one most frequently needed. These changes in the 
shape of the nose-pieces are readily effected by means of 
the pliers, especially if the nose-pieces recommended in 
Part I are used. When the guards are of cork, care must 
be taken that they are not scarred and broken by the pliers, 
and a special tool with a longitudinal groove in the jaws 
for grasping the sides of the nose-pieces is here of service. 

It will be readily seen, moreover, that the nose-pieces 
must incline equally to a vertical plane passing through 



the center of the nose; otherwise the glasses will stand 
awry; that is, provided the nose is straight and its two 
sides alike. In a large proportion of cases, however, a 
plane through the middle of the nose is only approximately 
vertical, so that one nose-piece must be inclined more than 
the other. (Fig. 59.) It is even necessary, sometimes, to 
incline the top of one nose-piece toward the vertical plane 
and the other away from it. 

The slope of one side of the nose from the crest backward 
is frequently steeper than that of the other side. A sym¬ 
metrical eyeglass on such a nose will stand with one lens 
close to the corresponding eye and the other standing for¬ 
ward away from the eye. One nose-piece will tend to 



84 SPECTACLES AND EYEGLASSES 

rest on its anterior edge and the other on its posterior edge. 
By partially revolving each nose-piece around the long axis 
of its bearing surface the glasses are brought parallel to the 
general plane of the face and each nose-piece presses evenly 
over its whole bearing surface. Fig. 60 is intended to 
illustrate what is meant. It is a diagrammatic view from 
above, showing the ends of the nose-pieces as arranged for 
a case in which the bridge of the nose does not slope equally 
on the two sides. These changes in the inclination of the 
nose-pieces are brought about by bending or twisting the 
foot at the point B in Fig. 61. 

Having conformed the nose-pieces to their bony support, 
the tension of the spring by which they are pressed against 



the sides of the nose is to be regulated, the object being to 
have just sufficient force exerted to keep the guards 
securely in place. If the latter are properly fitted the 
amount of pressure necessary is not great. Though this 
pressure should be evenly distributed over the surfaces of 
the nose-pieces, want of firmness in the (t pinch” of their 
tops is particularly fatal, as the lower ends then become 
the principal support of the weight of the glasses, render¬ 
ing them prone to topple forward and fall. To increase 
the tension of the spring, and consequently the pinch of 
the frames, the curve of the spring included between the 
lines at a, in Fig. 61, should be made more arched and 
rounded. Conversely, the force of the spring is lessened 
by flattening this arch. Any alteration in the shape of the 
spring, however, while it does not, of course, change the 
shape of the nose-pieces, does change the angle at which 












INSPECTION AND ADJUSTMENT OF SPECTACLES 85 

they are inclined to each other. For instance, if the spring 
be made more arched, the nose-pieces are brought nearer 
together, but the bottoms are especially approached 
toward each other. When the spring is flattened the 
bottoms of the nose-pieces are thrown proportionately 
farther apart than the tops. It follows that with each 
adjustment of the tension of the spring the inclination of 
the nose-pieces must be rectified. This is easily accom¬ 
plished by twisting the “foot” or support of the nose-piece 
at B in Fig. 61. 


A 



Fig. 61. 


When the points mentioned have been properly adjusted, 
the long axis of one or both glasses may fail to stand 
squarely across the face as it should do. The remedy lies 
in an appropriate bend of the spring at the point c (Fig. 61). 
This also requires a slight re-adjustment of the inclination 
of the nose-pieces to each other. 

The distance between the centers of eyeglasses is deter¬ 
mined (the distance between the nose-pieces when in use 
being a fixed quantity) by the distance of the nose-piece on 
each side from the center of the corresponding eye. The 
intercentral measurement may therefore be varied by vary¬ 
ing the size of the eye used, and by altering the distance of 
the nose-pieces from the edges of the lenses by an appro¬ 
priate bend of the foot B (Fig. 61), or by using a longer or 














86 


SPECTACLES AND EYEGLASSES 


shorter stud. The distance of the glasses from the eye is 
controlled by the length of the foot b, and in the better 
grades of goods this part is made in two or three lengths. 

Eyeglasses seldom stand too low upon the face but they 
frequently have the fault of standing too high, especially 
for near work. The neatest way of lowering them, but one 
which must be attended to when prescribing them, is to 
have the studs attached above the horizontal diameter of 
the lens, instead of at that diameter, as is usual. They 
may thus be lowered one, two or three millimeters; or 
a special form of nose-piece may be used. (Fig. 20.) It 
is sometimes necessary to combine these methods. 

The Care of Spectacles. —Spectacle frames will last 
longer and perform their function better if the wearer is 
instructed to exercise care in handling them. In putting 
them on and off, the hooks should be lifted from or into 
their position behind the ears; both hands being used, so 
as to avoid straining the temples widely apart or otherwise 
bending them. They should be folded together as little as 
possible, and when not in use should be laid in a safe place, 
open, and resting on the edge of the lenses, to avoid scratch¬ 
ing the surfaces of the latter. For cleansing them nothing 
is better than a piece of clean old linen, or, if very much 
soiled, a little ammonia and water may be used, except 
on cemented bifocal glasses. While cleansing, the frame 
should be grasped by the end piece and not by the bridge, 
and in replacing the glasses on the eyes care should be taken 
not to crush them against the lashes and thus soil the 
refracting surfaces at once. When cylindrical or prismatic 
glasses are worn, patients may return after a time with the 
statement that the spectacles are unsatisfactory, when the 
trouble will frequently be found to be due to bending of 
the frame; or a lens may have fallen out and been replaced 
upside down, or with the wrong edge inward. It is well 
to have such persons report periodically to have their 
glasses re-adjusted. 


INDEX 


Adjustment of eyeglasses, 82 
of spectacles, 76 

Airy, discoverer of astigmatism, 6 
Alhazen, 3 
Ancient glass, 1 

Angle, deviating, of a prism, 70, 74 
refracting, 70, 72 
Angle of crest of bridge, 59 
Apex of a prism, finding the, 69 
Assyrians, knowledge of lenses 
among, 2 

Astigmatism, discovery of, 6 
Asymmetry of the face, 82 

Bar spring eyeglasses, 34 
Base curve of toric lenses, 21 
Bifocal glasses, 27 

invention of, 7 
varieties of, 28 
Brewster, Sir David, 6 
Bridge, height of, 57 

relation of top of to plane of 
glasses, 46, 58 
width of base of, 59 
Bridges, manufacture of, 22 
varieties of, 24 

Care of spectacles, 86 
Celluloid frames, 11 
Cemented bifocals, 28 
Centering and decentering, 38 
normal lateral, 43 
normal vertical, 44 
of spectacles for constant use, 

43 

near work, 43, 44 
Centrad, 42, 71, 75 
Chemical composition of glass, 12 

87 


Component parts of spectacles, 11 
Conformation of nose-pieces, 82 
Crest measure, 60 
Crown glass, 13 

Cylinder, finding the axis of, 67 

Date of invention of spectacles, 4 
Decentered lenses, 38 

prismatic effect of, 40 
Deviating angle of a prism, 70, 74 
Dioptric system, 18 
Di Spina, Alessandro, 5 
Discovery of astigmatism, 6 
Distance of the glasses from the 
eyes, 45 

between temples, 79 
between the pupils, 36, 55 

Emerald used by Nero, 2 
Epicanthus, eyeglasses for, 37 
Eyeglasses, advantages and dis¬ 
advantages of, 9 
inspection and adjustment of, 
62, 82 

prescription of, 61 
varieties of, 32 
Extra front, 31 

Eye wires, manufacture of, 22 

Face, asymmetry of, 57, 60, 82 
Facing of spectacles, 47 
Focal length of lenses, 63 
Frames (see spectacle frames) 
Frameless spectacles, 22 
Franklin, glasses, 27 
inventor of bifocal 
glasses, 7 
Fused bifocals, 29 



, - 1 


88 


INDEX 


Geometrical center, 38 
Glass, ancient, 1 

chemical composition of, 12 
colored, 14 
optical, 13 
Ground bifocals, 30 

Height of bridge, 57 
Hook temples, 23, 79 

Index of refraction, 14 
Inspection and adjustment of 
spectacles, 63 

Interpupillary distance, 36, 52, 55 
Introduction, 1 
Invention of spectacles, 4 
Irregularity of surface of lenses, 76 

Kepler, Johann, 3 
Kryptok bifocals, 29 

Lateral centering, 43 
Lathe for grinding lenses, 18 
Lens cutter, 20 

oldest known, 2 
Lenses, decentered, 38 

finding focal length of, 63 
forms of, 14 

known to the ancients, 2 
material of, 12 
method of grinding, 18 
neutralization of cylindrical, 66 
spherical, 65 
sphero-cylindrical, 68 
numeration of, 18 
proving the strength of, 63 
shapes of, 25 
toric, 20, 50 
tilted, 46 

Lenticular segments, 28 
Locating the optical center, 68 
Lorgnettes, 9 

Maddox pupil localizer, 55 
Marking of gold spectacle frames, 
10 

Material of lenses, 12 

of spectacle frames, 9 
Meniscus, 15 


Natural position for spectacle 
bridge, 45 

Nero, concave jewel used by, 2 
Neutralization of cylindrical lenses, 
66 

spherical lenses, 65 
sphero-cylindrical lenses, 

68 

New system of numbering lenses 
18 

Normal position of spectacles, 38 
Nose-pieces, conformation of, 82 
Numeration of lenses, 18 
of prisms, 70 

Offset guard, 35 

Oldest known lens, 2 

Old system of numbering lenses, 18 

One-piece bifocals, 31 

Opticians’ lathe, 18 

Optical center, 38 

locating the, 68 

Pebble spectacles, 12 
Pencil for marking glass, 77 
Periscopic glasses, 20, 49 
Pince nez (see eyeglasses) 
Phacometers, 64 

Plane of the glasses, relation to the 
visual axis, 46 
Prism diopter, 71 

deviating angle of, 70, 74 
finding the apex of, 69 
numeration of, 70 
refracting angle of, 70, 74 
Prismatic effect of decentering, 40 
Prismetry, 74 

Prescription blank for spectacles, 
54 

of eyeglasses, 61 
of frame, 52 

Proving the strength of lenses, 63 
Principal axis, 38 
Principles of spectacle fitting, 38 
Pupil localizer, 55 
Pupillometer, 57 

Quizzing glasses, 9 


INDEX 


Refracting angle of a prism, 70, 74 
surfaces, 14 

Rigid frame eyeglasses, 34 
Rock crystal, 1, 12 
Romans, knowledge of lenses 
among, 2 
Rouge, 19 

Rule for measuring frames, 54 

Saddle bridge, 24 
Salvinus Armatus, 4 
Scratches, specks, flaws, etc., in 
glass, 76 

Segments, lenticular, 28 
Signs used in prescription writing, 
i7 

Spectacle eyes, sizes of, 25 
shapes of, 25 
fitting, principles of, 38 
frames, adjustment of, 76 
marking of gold, 10 
material of, 9 
prescription of, 52 
rule for measuring, 54 
Spectacles, bifocal, 27 
care of, 86 

component parts of, 11 
for cosmetic effect, 36 
date of invention of, 4 
early references to, 4, 5 
facing of, 47 
frameless, 22 


89 

Spectacles, inspection and adjust¬ 
ment of, 63 
invention of bifocal, 7 
patterns of, 22 
pebble, 12 
periscopic, 20, 49 
reversible, 24 
St. Jerome’s eyeglasses, 5 
Surface, irregularity of, 76 

Temples, distance between, 79 
manufacture of, 22 
varieties of, 24 
Tilted lenses, 46 

“Tool” for grinding spherical 
lenses, 18 

Tools for adjusting frames, 77 
Toric lenses, 20, 50 
Transparent glass found in Nin¬ 
eveh, 1 

Trial frames, 52, 62 

Ultex bifocals, 30 

Vertical centering, 44 
Visual axis, relation of plane of the 
glasses to, 46 

Width of base of bridge, 59 
Zylonite frames, 11 



















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